# Great Neapolitan Earthquake of 1857/Part II. Ch. IX

CHAPTER IX.
THE OBSERVATIONS AT POLLA AND ITS NEIGHBOURHOOD.
Upon the south and north slopes of the town, I clambered over heaps of stone and rubbish, and amongst entangled beams, ten, fifteen, and even twenty feet in depth, above the former surface of the place. Upon the eastern slope the ruin had been less; but over the larger portion of the city upon the hill, the destruction had gone so far, that objects suitable for the determination of the precise direction of the shock no longer existed. Viewed with a comprehensive glance, it was obvious that the shock had been in a direction not far from north and south, and had been very steeply emergent. The Photogs. Nos. 162 and 163 (Coll. Roy. Soc.) convey some faint idea of the general appearance of the more completely overthrown portions of the city; and those Nos. 164 and 165 (Coll. Roy. Soc.) of the character of the ruins seen from the midst of them. No. 164 is a street of the more level part to the east of the city; No. 166 (Coll. Roy. Soc.) is amongst the heaps that overwhelmed the site where the church Santa Trinita had stood; and No. 165 looks upwards over a quarter of a mile of ruin, towards that of the Castello that ​
| Photo Pl. 164 | Photo Pl. 168 |
| Vincent Brooks, lith. London. | |
| Monastery of St. Claire, Polla. | Church of the Madonna Di Lorreto, Polla. |
| Looking South West. | |
Wherever the walls had stood sufficiently, to observe the direction of fissures, they were found traversing the former at angles indicating steep emergence, as may be seen in the three last Photogs. In general, however, this angle approached the vertical more, at the upper parts of the city, than at or near the base of the hill on which it stood, proving that whatever had been the angle of emergence of the wave, at the base of the hill, or on the level of the plain, the hill itself, short and stumpy though it was, had vibrated with a proper motion of its own, which, being necessarily nearly horizontal, had thus modified the angles of the fissures. Descending by the east slope, and amongst the buildings around the base of the hill, and towards the bridge over the Calore, I found abundance of objects for fixing the direction of wave-path.
The church of the monastery of St. Chiara (Photog. No. 167) stands about half-way down the slope. The plane of the west end wall is exactly cardinal (north and south), and shows large diagonal fissures running quite across the façade. The direction of these proves a wave-path, north to south 10° to 12° E. of N., and an angle of emergence not less than 40°. The support of the solid belfry at the S. W. quoin, has prevented fissures crowing each other in this end, and, with the exception of one great diagonal shattering, at the top of the belfry, its construction, has involved chiefly a deep vertical fissure in it, extending almost to its base, but at the east end. The diagonal fissures of the main building, cross and have their upper ends inclined, chiefly towards the north. The roof has in great part ​fallen in, and such portions of it as still indicate the direction of throw, prove it to have been from S. to N., at an angle downwards of 50° to 60° with the horizon. Two small and slender iron crosses, one on the top of the pediment over the west end, and the other on the belfry top, are bent over, the former to the north to about 15° from vertical, the latter to the south, about 25° from same. I found I could not reach these, to get the scantlings of the iron, &c., as measures of velocity; the ruins continuing to fall at unexpected intervals, and the height being considerable. The belfry cross, by the theodolite telescope micrometer, appears to be about 21/2 feet high, and of iron, about 13/8 $\times$ 3/8 in. section, and is bent on the flat.
The church of the Madonna of Loretto was a solid Corinthian structure, still lower down, and in great part built of brick, with heavy semicircular arches, to the nave and aisles, and a heavy semicylindric roof; its axial line cardinal. As may be seen in Photog. No. 168, it is fissured down to its base, the fissures (some of which, of a regular and measurable class, may be observed under the altar-piece at the N. E. corner low down) all indicate a wave-path from north to south, and in direction about 160° 30′ W. of north, and an emergence of 50° to 60° with the horizon. The north flank wall, leans heavily out towards the north, as do all the large sashes still standing, high up in that wall. The main mass of the rubbish of the fallen roof, is in the inside of the church, and towards the north side of the floor. The shear or break of the roof vault also proves the direction of the force of fracture to have emerged from the north at a steep angle, as may be seen in the Photog. No. 168. On the right side near the pulpit, may be seen ​a wooden Roman-Doric pillar, the base of which was fixed into the flagged floor, and which carries on its top a wood globe surmounted by a small gilded cross. The two last are bent over upon the iron spindle that confined both, towards the south in a plane very nearly north and south. This probably was produced by the second half vibration of the wave, but may have resulted from a blow from some falling body. The spindle or bolt that held ball and cross, was about 5/8 in. diameter; the ball about 8 in. diameter; and the cross rose about 15 inches over it. I could not get its weight, but both it and the column were of hard wood. This church was built after the earthquake of 1652, and with special reference to future shocks. Iron chain bars had been built into the arches and roof, and would no doubt have done good service had the shock been more horizontal, but its direction of steep emergence took them transversely, and in several instances tore them across. The fractured ends of one, may be seen projecting from opposite sides, of the fallen roof-vault, and corroborate all other evidences of the steep emergence of the wave here.
Photog. No. 169 (Coll. Roy. Soc.) shows the interior of the church of St. Dominico (?), attached to a monastery a few hundred yards from the last, and looking westward. This was a much worse built church, and of stone. The direction of the inclined fissures may be seen, inclining northwards at their upper ends, in the west wall. The line of fracture of the arched groin of the roof at the south side, as well as the mass of the fallen material of the roof thrown towards the opposite or north wall, all prove the same general direction of path and steep emergence.
Photog. No. 170 presents an arched wall in a north and ​south plane, 20° E. of north, in the same monastery, and an example, of heavy dislocated, and inclined fissures. The average angle here was 45° to 50° from the horizon. On principles explained in Part I., this is below the actual angle of emergence, as here each pier rocked more or less individually, and with the direction of the voussoir joints tended to give greater perpendicularity to the fissures. The crown of the upper tier of arches had all fallen in, and they, as well as the cross wall seen to the left of the picture broken off, had all been thrown northward, as the marks of abrasion upon the plaster of the face of the arched wall testify.
The Photog. Fig. 171 (Coll. Roy. Soc.) is an example of a frequent way in which arches are affected, indicating steep emergence. This arch is in a wall not far from north and south in plane. The ink lines $cc$ point out the extremes of minimum and maximum angles of emergence of the wave coming up from the northward. The dislocation of the arch produces loss of momentary support to the crown, the top ring of which has descended some inches, bringing a mass of wall along with it; but the fissure closing upon it at the second half vibration of the wave, has caught it, and the arch ring remains, nipped as shown, having descended through the space below its original intrados, at the point of intersection of the fissure, in rather less than the time occupied by one complete vibration of the wave. The data are not precise enough, however, to found calculation upon with any certainty as to this short interval of time.
The Photog. No. 172 (Coll. Roy. Soc.) looks N. W., and shows several tiled roofs fallen in, and is an example of the ​
| Photo Pl. 174 | Photo Pl. 173 |
| Vincent Brooks, lith. London. | |
| House on the Bank of the River. Polla. | Small House by the River. Polla. |
| Looking Eastward. | Looking Westward. |
​general character of roof fall, here and wherever the angle of emergence is steep.
Descending still to the level of the plain, but not far from the hill of Polla, I found the most instructive examples.
The Photogs. Nos. 173 and 174, are views of an isolated sluice-house, that stands upon the east or left bank of the Calore, and was intended for use in relation to irrigation works, connecting the river channel with an artificial one at certain seasons.
In No. 173 the east side and north end of this structure are shown, and in No. 174 the west side and south end. The building is quite modern, constructed of good rubble masonry, with cut limestone quoins and jambs, flat window arches, and stone cornice over entablature. It is about 30 feet high above the soffit of the sluice arch, 30 feet wide north and south, and about 20 feet wide east and west, the walls 2 feet 6 inches thick. The building is exactly cardinal, its longer axis N. and S. Referring to No. 173, heavy inclined fissures will be seen running from both quoins, and meeting near the centre at top. The wall above the window arch is dislocated, and the voussoirs are thrown downwards, by a force emergent from the north, and nearly parallel with the fissure $ca$. Several minor fissures, which do not show in the Photog., existed through the arch joints near $c$, at 45° and 55° from the springing (or horizon), and in nearly the same direction, diagonally through the opposite or southern pier face.
The north end wall (to the right in Photog.) is scarcely fissured at all. The whole of the blocks of stone of the cornice at the north end, have been thrown from off the ​top, and falling not due north, but some points to the east of north, upon the brow of the bank below, have rolled down into the dry bed of the river, and are seen lying about under $c$. The cornice blocks tailed in upon the wall, but a short distance, and were not far from being balanced over the front arris of the entablature, on which they were bedded. The force that overthrew them, therefore, was one emergent from the north towards the south, a few points to the east of north; and from the position of the centre of gravity of the course of stones, it could not have been of steeper emergence than 70° to overthrow them, a line from that centre, at that angle, cutting the entablature arris. The roof had fallen almost completely in, and its debris of tiles and short timber, lay chiefly towards the north end of the interior.
Referring to Photog. No. 174 of the west side, where the wall was much more uniform, as to aperture, &c., the great line of fissure will be observed, inclining at top towards the north, and crossing through the arch lintel of the doorway about the centre. The angles of these fissures were 30° to 34° with the quoin, giving an emergence of 60° to 56° from the northward. The cornice was all perfect on the south end, as also on the east and west sides, except where fractured over the window at the east side, thus corroborating all the other proofs of steep emergence from the northward.
A number of the entablature stones, laid with wretchedly shallow beds, (only veneered upon the face of the rubble hearting,) on the centre part of this west side, are seen fallen out. They were lying very near the base of the wall, on the roadway that is seen here, passing over the ​sluice and in front of the building above it, and they had been slightly projected, by the eastern element of the emergent wave, at the moment when they were relieved from the detent of the cornice above, by the giving downwards, of the central part of the wall over the door lintel, which will be observed also down. The south end was very little fissured, and the evidence was clear of a wave-path, not varying above 15° or so from the plane of the side walls, or north and south.
The fissures were 2.25 inches wide at top, on the west side, and about 2 inches (in the same direction), on the east, which also indicates an eastern element of about 17° in the wave-path.
Referring back to Photog. No. 173, in the foreground certain piers are seen—groove piers for inserting stop gates, to pond the water for irrigation in the dry season. These stood in about 4 feet water, were of sound dressed limestone ashlar, and presented the long way of their horizontal section to the wave-path. Each pier was about 10 feet long by 4 wide, and about 12 feet high; not a stone was dislodged. The top courses were cramped together. From their peculiar narrow figure a very moderate divergence of the wave-path, from the north and south, would have probably sufficed to dislocate, if not to overthrow them.
Several other buildings of rather large size, situated about this portion of the city, chiefly ordinal, gave by fissure, when reduced, a wave-path varying between 157° 30' and 164° W. of north. It would be unprofitably tedious to give the details at length.
One of the most instructive buildings at Polla, was the ​Palazzo Palmieri, situated not far from the bridge over the Calore, and nearly on the level of the plain. The west front of the house (i.e. looking eastward) is seen in Photog. No. 176 (Coll. Roy. Soc.), the central building in the picture, and to the left of it is the Capella Palmieri, forming a connected building with the palazzo, and with its west end ranging with the front of the latter.
In Diagram No. 175, Fig. a, is a sketch elevation of this west front and block plan of the building. The house is exactly cardinal; it is large, comparatively new, and tolerably well built, though of short lumpy stone, with much cut limestone about the quoins and jambs, &c. It consists of a central court, surrounded by buildings of two stories and an attic, beneath the tiled roof. It is built upon the solid limestone rock, which at this point rises up through the deep alluvium, that lies around it at every side for a considerable distance. The ground is level around it except on the south, where it slopes off rather rapidly, the rock disappearing beneath the alluvium. It is, in fact, built upon the top of a sort of low pinnacle of limestone rock, that comes up (like many others round the borders of the great plain) to its level, through the deep alluvium, so that it stands as it were on the top of a little subterranean "colline," like a rock in an ocean of earth. The beds of the limestone here, as generally throughout the Vallone, are almost vertical, or at extremely sharp dip, and tend to a general east and west direction of strike.
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The Palazzo Palmieri is fissured diagonally in every wall more or less, those in the north and south walls being the most formidable. A large wedge-shaped mass, carrying with it a portion of the roof, is thrown from the S. W. quoin of the front, and a large portion of the south external wall, is prostrate and thrown to the south.
In Fig. 1, Diagram No. 175, the form, position, and angles, of the principal fissures found in the west front are shown, looking eastward, entering beneath the "Portoue," and looking back or westward. The fissures formed above the archway in the north and south wall of the interior façade, parallel to the front, are seen in the Photog. No. 177, and in the sectional sketch (No. 178) taken on the line $\mathrm {AB}$ (on plan), and looking westward.
The south external wall, $e$ to $g$, had been thrown to the south, and the upper part lay between $t$ and $k$.
This is shown, in part, in Photog. No. 180 (Coll. Roy. Soc.), as seen from the balcony of the staircase, directly opposite the entrance gateway, and is shown in elevation in Photog. No. 179, looking eastward. In all these the general directions and angles of the fissures are correctly exhibited. There are some fissures, in all the walls, but the great mass are in those running north and south. ​These make angles with the vertical of 30° to 40°. They cross in both directions, but the larger and greater number, are inclined at top towards the north.
The fissures in the walls running east and west are all more nearly vertical, than those in north and south walls. They prove that the building rocked in all directions, but that the main force was one emergent from the north towards the south, at an angle with the horizon of between 50° and 60°. The violence of movement in this direction was great, where, for example, one fissure passed through a joint of the arch, composed of long, curved, ill-formed voussoirs, at $s$ (Fig. 178), over the front gateway, the arrises of the stones at the junction, are flushed off, as seen in enlarged Sketch (Fig. 178), and many such occur in the arches of the stone staircase, a square winding one, which, from its irregular form, is dislocated in every direction, as may be partly seen in Photog. No. 180 (Coll. Roy. Soc.) of the west interior façade.
The external wall $eg$ at the south end, $\mathrm {B}$ on block plan, was built on scarped ground below the floor level. It had been about 38 feet in height, and was but little restrained by cross walls, and these only at the ends, about 70 feet apart, and was built of bad rubble (as is much of the Palazzo, though with costly cut-stone dressings, &c.), in lumpy blocks, of 10 to 16 inches greatest length, the mortar joints thick and bad, and no thorough bond. This wall was 2 feet 4 inches thick, and about 8 feet in height from the top, $e$ to $g$ (Fig. 178), was thrown outward, so that the great mass of the debris lies, in a parallel heap upon the sloping ground between $k$ and $t$, from 10 to 18 feet from the foot of the wall, the average distance of ​throw being 14 feet, and the height from which it had descended 30 feet, i.e., these being ordinates to the centre of gravity of the thrown mass. The portion thrown off from the top, was projected during the second semiphase of the wave, by the velocity impressed during the first, or by the return stroke of the shock.
Let $a$ = 30 feet, $b$ = 14. Then from the equation
$\mathrm {Tan\,} e={\frac {2\mathrm {H} \pm {\sqrt {4\mathrm {H} \left(\mathrm {H} +b\right)-a^{2}}}}{a}}$
we find tan e = 1.593, and e = 57° 50′, an angle of emergence which is within the limits given, by the fissures (which in this instance are extremely well defined in the Photogs. Nos. 177, 179, and 180, Coll. Roy. Soc.) in several parts of this building, and in that of the sluice-house at the river close by.
I shall recur to this wall when treating of the "camine," thrown in the dining-room.
Throughout the Palazzo, the floors are of beton and tiles, laid upon thick oak planking, crossing over oak and fir joists at about three feet apart: some in the largest rooms at the north wing, have been brought down altogether, the fractured beams showing, that they yielded to the inertia of the mass of beton and tiles, under the emergent wave. All are fissured, in various directions diagonally across, the lines more or less curved, with the hollow sides of the curves towards the centre of the floor, and upon the whole making angles of about 22° 33′ with their respective walls, and crossing each other towards the mid length, nearly at right angles; the great prevailing direction 22° 30′ E. of north, and at right angles to the same. These floor fissures, as nearly as they could be drawn by the eye, ​in one of the large reception rooms of the first floor, are shown in Fig. 181.
$ac$
The Palazzo though thus shattered, and in many places dangerous to approach, had yet several articles of heavy furniture, &c., remaining untouched and unremoved; and through the politeness of one of the Palmieri family, ​who attended me with his servants, and answered all my inquiries, I was enabled to make a more minute examination of the interior of the house, and to record some most instructive cases of disturbance by the shock, giving measures at once of wave direction and of velocity.
Of these, one of the most valuable in deduction, is the overthrown stone and brick breast, of the "camine," or chimney hood, of the kitchen adjoining the great dining-room. This room was in the north wing of the building. The chimney hood was built, against the south side, of a wall running nearly east and west; so that a normal to its face, was north 10° E.
The chimney breast, which is accurately represented from careful measurements, in b, &c. Fig. 175, consisted of three blocks of Apennine limestone; two being vertical side jambs, and the third an imposited lintel, with a little brickwork superimposed, to complete the junction with the flue in the wall.
Fig b, 175 is the "camine," in front elevation, Fig. b, &c., 175 in ground plan, Fig. b, 175 in vertical mid section transverse to the line of wall. The dotted lines, show it as it stood before the earthquake, the hard lines representing the respective positions, in which I found the three blocks of stone lying, on the floor with the loose brick and plaster rubbish (chiefly) in the midst just as they had fallen; and Photog. No. 183 represents their appearance, and that of the wall from which they had been separated. The three blocks of stone merely stood against, the face of the chimney recess of the wall, and were made good to it with mortar, as was also the bit of pyramidal brickwork above the lintel. There was no bonded connection between them, nor any ​real cohesion, as the smoked surfaces of the faces of junction proved, that the expansion and contraction of the limestone blocks by change of temperature (when fires had been lighted) had completely severed the mortar union between them and the wall.
The whole fabric stood, therefore, ready to be overthrown by any force competent simply to overset it upon its base, in a southerly direction. The Figs. b, &c., 175 show that the whole mass had canted over to the south, turning upon the front lower arrises, of the side or jamb blocks, the latter carrying the lintel, for a short portion of the arc of descent, upon their upper ends. The brickwork above, owing to the position of its centre of gravity with reference to its base upon the lintel, when freed from the face of the wall, and the small velocity with which the mass must have been overturned, had fallen on the central space of floor, between the three stone blocks. The lintel block, had landed upon the floor, close to the upper ends of the side jambs, and had then, partly by the direction of descent, partly by the effects of its own elasticity, and that of the beton and tile floor, either fallen over, or slided (or both), forward a few inches further south, having been broken in two by the stroke on the floor, at a soft joint in the limestone. The two side jambs, from their peculiar form in horizontal section, and their setting, at an oblique angle to the face of the wall, had canted round the inner angles of the front arrises at their bases; and in their descent, the moment they got free from the lintel, had turned round partially, upon an axis parallel each to its own length, and lay, after thus rotating through about 90° each, with its east face uppermost, both having rotated towards the west; so that when fallen, $E$ was ​removed further west from the centre line of the "camine," and $F$ in the same direction nearer to the same line. The centre of gravity of the whole lintel block, in the same way was posited, at a point a little to the westward, of the vertical plane normal to the face of the wall, and passing through it as it had stood erect. The inner face (or cove) of the lintel, was uppermost as it lay on the ground, proving that it had simply been thrown down, without any secondary disturbing force.
Now this overthrow might have been produced either by shock from the south to the north acting upon the mass by its inertia, or by a projecting force, of a shock from north to south, carrying the wall along with it, forcing forward the chimney breast, and then at the return stroke, (or second half of the wave,) projecting it towards the south. Each assumption would lead to different results; but as there exists abundant and quite independent evidence that the general direction of wave movement at Polla was one from north to south, and not the contrary, we must conclude that this "camine," was projected, and thrown by the forward stroke of the wave. The rotation of the side jambs, and the position of final deposit of the lintel, prove that the direction was somewhat, from the east of north to the west of south, and geometrical considerations based on careful measurements on the spot, prove the wave-path to have been 165° W. of north which produced the overthrow.
Referring to Eq. IV., Part I., we can adopt the fall of the pieces of this "camine," as a means to determine, both the angle of emergence, of the wave-path here, and the velocity of the wave particle at its maximum.
The lintel $G$ moving through a very small arc at the ​commencement of its motion, may be assumed to have moved horizontally and free of restraint from the jamb stones. The height of its centre of gravity from the floor, was 4.33 feet; but inasmuch as the small bit of pyramidal brickwork above it, was separated and began to move along with it, we may consider the centre of gravity of the lintel raised by it to 5 feet. The block was thrown from this height to a horizontal distance of 7.2 feet, including that of its having once turned over, towards the south upon its front arris on reaching the floor.
We may further conclude, that as it so came to rest, the velocity impressed, at the first moment of its motion in an horizontal direction, was not greater than sufficient to make it overset upon that edge.
The difference of the side and diagonal of the lintel (in transverse section) = 0.62 feet; 4/3 of this is = $H$, the height due to the horizontal velocity of overthrow.
Then
* $b=5{\text{ feet. }}a=7.2{\text{ feet}},$
* $-b=a\tan {e}-{\frac {a^{2}}{4\mathrm {H} }}$
* $\mathrm {Tan\,} {e}=1.472$
$e=55^{\circ }49^{\prime }$
But $V^{2}=2g\mathrm {H}$, the horizontal velocity; the total velocity, or that in the direction of the wave-path is therefore
* $\mathrm {V} ^{2}=2g\mathrm {H} .\sec ^{2}{e}$
* $\mathrm {V} =12.863{\text{ feet per second.}}$
​If we apply this same method, to the jamb stones alone, or, as above, to the lintel alone, or to the whole viewed as a single mass, we arrive at the same value for $\mathrm {V}$, within extremely narrow limits.
The assumption upon which this method depends (as respects the horizontal velocity impressed) is open to the objection of being slightly arbitrary; whether by compensation of errors however, or not, the result arrived at is extremely near to the truth, as will appear further on, and is controlled by the following calculation, which is open to no such objection.
Taking the overthrow of this "camine," in connection with that of the wall $\mathrm {B}$, at the south end of the palazzo from which the upper part was thrown off. We have here two different bodies at the same spot, projected by the same shock, and by the same phase (the second) of the wave; and we can apply the method developed in Part I. (Eq. XL. to XLVI.) to determine both the emergence of the wave-path, and the velocity of the wave particle, in that path.
$a$ and $a^{\prime }$, $b$ and $b^{\prime }$ being the respective horizontal and vertical distances of projection, we have
* $-b=a\tan {e}-{\frac {a^{2}}{4\mathrm {H} \cos ^{2}{e}}}$
* $-b^{\prime }=a^{\prime }\tan {e}-{\frac {a^{\prime 2}}{4\mathrm {H} \cos ^{2}{e}}}$
* $\mathrm {Tan\,} {e}={\frac {a^{2}b^{\prime }-a^{\prime 2}b}{aa^{\prime }(a^{\prime }-a)}}$
* ${\text{and }}\mathrm {H} \cos ^{2}{e}={\frac {aa^{\prime }(a^{\prime }-a)}{4(ab^{\prime }-a^{\prime }b)}}$
​and substituting for $\mathrm {H}$ its value ${\frac {V^{2}}{2g}}$
* $V=g\times {\frac {aa^{\prime }(a^{\prime }-a)}{2\cos ^{2}{e}(ab^{\prime }-a^{\prime }b)}}$
* $a={\text{ 7.4 feet, at the final point of repose}}$
* $b={\text{ 4.33 feet}}$
* $a^{\prime }={\text{ 14 feet, }}b^{\prime }={\text{ 30 feet.}}$
* $-4.33=7.4\tan {e}-{\frac {13.69}{\mathrm {H} \cos ^{2}{e}}}$
* $-30=14\tan {e}-{\frac {49}{\mathrm {H} \cos ^{2}{e}}}$
* $\mathrm {Tan\,} {e}=1.357{\text{ and }}e=53^{\circ }37^{\prime }$
$V_{,}=$
* $\mathrm {V} =V_{,}\sec {e}=13.490{\text{ feet per second.}}$
The difference between the former calculation and this is = 0.627 feet per second, or little more than half a foot per second. If we take the mean of the two determinations we find
* $V=13.176{\text{ feet per second.}}$
$a$
$b$
A chimney hood of brickwork standing over the charcoal hearths in this kitchen, was at the end wall of nine inch brickwork $\mathrm {A}$, (Fig. 185, Sketch Coll. Roy. Soc.) started out from the same line of wall, by about 21/2 inches at top, in a height of 12 feet, but was not overthrown. It was propped however, by the return portion of the hood, at right angles to it, and so cannot be adopted for calculation, although affording a rude measure, that the velocity must have been small. In this same kitchen, the naked oak planking, of the ceiling or floor above, brown with wood smoke, shows where the ends of the boards have been drawn from their insertions in the walls, in the direction of their own length, and therefore transverse to the north and south joists, upon which they were laid, but which have not been moved, but to which the planks were not spiked or trenailed. The mark of the white mortar, shows the draw or shove to have been from north to south; it is 23/4 inches at the west end, and 4 inches at the east end, of the east and west wall, in a length of 25 feet; and as the normal to this wall bears 10° E. of north, the horizontal direction of wave-path deducible from this is as before, about 165° W. of north.
The actual amplitude of the wave in an horizontal direction cannot have greatly exceeded the average amount of shove, of these heavy beton and plank floors, and hence cannot have much exceeded 3 or 4 inches here.
In another room stands in a corner, against the wall $\mathrm {T}$, which ranges north and south 10° E., and abutting upon the wall $\mathrm {S}$ at its southern end, a very heavy oaken household ​press (Fig. 184, Sketch, Coll. Roy. Soc.) It was 10 feet 6 inches high, 9 feet 6 inches long, and 2 feet 2 inches wide from front to rear. It was not thrown down by any east or west movement, but was shoved, by a north and south movement, along the beton floor upon its eight stumpy oaken feet, each 4 inches square (surface of contact with the floor), in a direction towards the south, 13/8 inch from the north wall against which it abutted.
The owner informed me, that it had been quite full of household articles, pretty uniformly distributed on its shelves; that it had not been filled so as to be top-heavy, or unequally loaded; and they estimated the total weight at about 800 rotuli = 1552 lbs. avoir. A quantity of china which occupied the upper third in height of its two southern divisions, was all found broken and thrown into a heap, at the south end of the shelves. A low velocity horizontally, not exceeding 2 feet per second, would have sufficed to make this press slide that distance upon a smooth floor, which would be only a little above 31/2 feet per second in the wave-path. It must therefore have been arrested by some inequality in the floor, or by its feet ploughing into the beton, and will give no certain result, except that the velocity was sufficient, to dash the contained china first north by the first phase, and then south by the second phase of the wave.
In one of the great drawing-rooms there stands still in situ, a very ponderous cabinet or chest of drawers of walnut (Fig. 186, Sketch Coll. Roy. Soc.), with its back against a wall running east and west, and free to fall to the south. It was quite full in every part with house linen, and estimated to weigh 400 rotuli = 776 lbs. avoir. (cabinet and contents). It rests on four irregularly-shaped feet formed by ​perforations in the front, ends, and back, and was neither overthrown nor shoved out from the wall at back.
Referring to the Equation V., Part I.,
* $V^{2}={\tfrac {4}{3}}g{\sqrt {a^{2}+b^{2}}}\times ({\frac {1-\cos {\theta }}{\cos ^{2}{\theta }}})$
$\theta =33^{\circ }{\text{, }}\alpha ={\text{ 4.75 feet, and }}b=2.80{\text{ feet, }}$
$V$
* $V=7.378{\text{ per second}}$
$e$
${\text{V}}$
* ${\text{V}}=V\sec {e},$
* ${\text{V}}=13.1328{\text{ feet per second.}}$
${\text{V}}$
${\text{V}}$
The maximum velocity of the wave vibration at Polla may therefore be concluded with confidence to have been in round numbers 13 feet per second. The mean velocity obtained from both calculations, 13.176 feet per second, is a little more (0.043 feet per second) than sufficient to have overturned this cabinet; but the slightest inequality in its loading, or in its parts, by altering the height of the ​centre of gravity, would have sufficed to prevent its overturn, unless by a velocity greater than that assigned, by more than the difference as above.
Everything at Polla consentingly proved a wave-path of steep emergence, and from the north, or very nearly. The result, united with my observations in the upper part of the valley of the Tanagro, proved that the wave-path, as detemined at Naples, either belonged to a separate focus altogether, or must have some complicated relation of disturbance, by reflection or otherwise, with the focus I was looking for, and which it now began to appear, I should find somewhere to the north, and not very far east or west of the meridian I was then upon. I therefore dismissed from my thoughts for the time the irreconcileable phenomena at Naples, leaving it to further observation to solve the apparent enigma.
The Judice of Polla, Signore Ferdinando Ganuzzi, politely accompanied me over the place. He had been at Polla on the night of the earthquake. According to his statement, confirmed by that of the Syndic, and others of the town, they were suddenly alarmed by the rushing sound, "Mormorio buccihaute rapidamente;" and almost instantly, while it yet was heard, the first great shock came—"sussultatorio," succeeded in a very few seconds afterwards—how much they could not say, probably 10″ to 20″—by another movement, that some of them habitually spoke of as a second shock, which was "undulatorio." But there was no such distinct interval of quiet, and arrival of a second shock, both being oscillatory, as was felt at Naples. The Syndic was on the second floor of his own house, which did not fall immediately; the first ​movement was sufficient to cause him to lose his balance when standing, and to fall upon the floor. He found some difficulty, owing to the second undulatory movement, in regaining his feet to fly from the shattered house.
No correct observations were made as to the time of the shock: all was confusion and alarm. He noted his watch, and by it the time was 10$^h$ 15$^m$ solar time, in Frankish hours. But he admits that all their watches and clocks are regulated by the setting sun, and are not reliable within narrow limits.
There was a second shock of considerable violence about an hour after the preceding, which shook down many buildings, that had been shattered by the first.
The Capo D'Urbano, a very intelligent man, made a curious and probably not unimportant remark, as to his experience of the sound. It seemed to him to reach him "through his legs, as he stood up," although, he added, "it was everywhere." This suggests the probability, that much of the sound in earthquakes may reach the auditory nerves, by transmitted vibration from the ground or other solid objects, through the bony skeleton; just as when a poker held by a string to the ear is struck, and thus may convey from a very small vibration an overpowering sense of sound to the auditory nerves.
I took from the centre of the middle arch of Polla Bridge, several intersections to correct magnetic declination by.
Monte Corticata bears . . . . . . . . . 170° W. of north. Tower of Diano (highest tower) . . 179° W. " Atena (high tower) . . . . . . . . . . . . 156° E. " La Sala . . . . . . . . . . . . . . . . . . . . .152° E. " Pizzo di Cirazzo . . . . . . . . . . . . . . 179° E. "
​These give a declination varying between 13° 30' and 15° 30' west. I also took an observation of the sun, but on computing I find that some decisive error must have occurred in my note of the sun's azimuth, which renders the observation valueless, as there was no ground for supposing any material disturbance of the ordinary amount of declination at this spot, more than at La Duchessa, where the same error appears to have been made.
As I left Polla in the afternoon. the grand plain of the Vallone di Diano opened before me, level almost as a sea, of deep rich alluvial clays, which, as they approach the roots of the mountains that rise almost abruptly from the plain, assumes the form of a sort of sloping and almost continuous terrace all round the plain, elevated everywhere a few feet above its average surface, seldom rising more than 40 feet above it, frequently not half that.
The outline section of this terrace, as it sweeps towards the centre of the plain, is that of a flat, hollow, parabola-like curve, with a slope varying from 15 to 1 to 60 to 1, or even still more gradual, and suggests at once to the eye, the geologic conditions that produced the piano; once a large lake, or arm of the sea, which found egress for its waters at a level probably not lower than the summits of the Campestrina Pass. In that condition of things, the rich bed of mud was deposited, that now forms the basis of the valley and of its agricultural wealth.
The great fracture, through which the Calore now finds its course sub dio, must have been subsequently formed, and through it the lake was drained down to the level of its marginal terrace. Its subsequent progress of desiccation must have been gradual, and dependent upon the rate ​of erosive deepening of the river at its north extremity; and for ages it must have remained a shallow and probably pestilential lake, of above 20 miles in length and 4 or 5 miles wide. The total difference of level now, between the water of the Calore at Polla and at the junction of the Peglia with it at the extreme southern end, is said not to exceed 6 or 7 feet by Signor Palmieri of the Corps of Strade e Ponti, or only 3 inches to the mile. The marginal terrace round the plain contains numerous fragments of limestone, some angular, many more or less rounded. But the great clay central bed scarcely presents a pebble until high up towards the southern end of the valley, where the washings of the hill-side torrents disclose coarse gravel and boulders also, embedded in it. On the west side of the valley, I pass St. Arsenio and St. Pietro, both low-lying villages, placed upon the level of the marginal slope, that like a great shallow saucer, surrounds the plain. These towns have suffered but slightly, although not five miles in a right line from Polla, and St. Pietro, the more distant, has suffered the most. The damage done, however, is almost confined to old houses, built of the usual sort of wretched, short, nobbly, bondless limestone rubble, of rounded lumps like irregular loaves, of from 6 to 16 inches diameter, with thick joints, of bad mortar, made of clay rather than sand; the general direction of wave-path from 142° to 144° W. of north.
The character of the limestone of the mountains at both sides of the valley, begins rapidly to change, from that of the valley of the Tanagro and Salaris. It is no longer the hard, sharply-fracturing, clearly-bedded, and well-defined stone of the latter, but a loose, irregular, ill-defined, and ​inarticulately-bedded, crumbly stuff, extremely like in lithological character, the white and variegated limestone beds, of great portions of the upper limestone of Roscommon, Leitrim, and King's County, in Ireland, but much more sandy and siliceous.
Such bedding as is traceable upon the surface, appears in a direction almost vertical, and although with many changes of strike, having, upon the whole, directions ranging transverse to the main axis of the Vallone; and hence, upon the whole, presenting the flat of the bedding, to the direction of the wave-path along the valley. There is no general indication of continuity of bedding or of formation, at the opposite sides of the valley, nor anything that decisively points out, whether this limestone of its flanks continues right across beneath its clay filling or not. It seems highly probable, from the general structure of the country, however, that it does so; and equally so, that limestone, breccia, and marl beds, may lie between the crumbly limestone and the alluvial clays. The more distant and higher summits, visible clearly by the telescope—particularly those on the east behind Atena—present more of the character of the hard limestone of the Tanagro. The limestone of the lateral hills is so ill compacted, and so easily acted on by air, water, and carbonic acid, that it is cavernous in every direction, and weathers into holes and pits of fantastic forms. In hand specimens, its structure frequently shows it to consist of a mass of compacted crumbs, of rather harder limestone, angular, and not much unlike in size and form, Vesuvian "lapilli," but very slightly coherent by a softer calcareous paste.
This form, gradually passes into an almost white ​cretaceous-looking limestone, very soft, friable, and filled with disseminated fine white sand, in many places, particularly along the east flank of the Vallone. The hill sides everywhere present strong evidences of denudation, down to within three or four hundred feet of the level of the plain. Their summits and flanks are all swept almost bare, and the great deposits of detrital material above the plain, are only found in hollows, where it was entrapped, or in steep banks not elevated far above it, and now rapidly eroding by torrents. Almost all the scattered buildings that I pass along the plain, going southward, present indications of a general wave-path from north to south, varying some points east or west of that. On the east side of the great military road, however, about a mile from Polla, are some isolated buildings, not more than 150 feet above the plain, yet upon the solid limestone, which present decided east and west characteristics of wave-path—one especially, a nearly new building, having a wall of about 50 feet in length running north and south, and connected with others nowhere but at its extreme ends, by two transverse, or east and west walls. About 7 feet in height is thrown off the top of the north and south wall for its whole length, and thrown towards the west and south, while the east and west, or end walls, are merely fissured slightly. The original height of the fallen wall was about 35 feet, its thickness 2 feet, and the mortar was not yet indurated. The debris had rolled down a slope of 1 in 6, and much of it was 20 feet from the base of the wall, and hence did not admit of any calculation as to angle of emergence.
It was obvious that this was some local disturbance connected with the rocky range behind (to the east), but ​what I could not at the time discover. At about 2 miles south of Polla, and at a point of the road about the general level of the central plain here, I took by barometer the level of the Vallone. At 1$^h$ 20 Greenwich time (14th Feb.) barom. reads 29.6 inches, thermo. 62°, which, reduced, gives 581.5 feet for the level of the central plain above the sea. Either by misreading, or through some great inequality of atmospheric pressure on this day, between Naples and the Vallone, this observation is certainly about 100 feet below the truth; for the bed of the Tanagro at Auletta was ascertained to be 576 feet above the sea. The waters, issuing slowly from St. Michael's cavern, are about 80 feet above the river beneath, and there is probably 20 feet fall between Perrosa and Auletta; while it is probable that the fall in the rocky tube is but a few inches between the water at Polla and its issue at the cavern. There must therefore be a fall between Polla and Auletta of about a hundred feet. We may therefore take the mean level of the Vallone di Diano at about 700 feet above the sea, and the barrier of Campostrino, before its fracture, stood 1,363 feet above the plain. This was the depth of the ancient lake or arm of the sea.
While at Auletta, the gendarmes and others there, had affirmed that on the night of the earthquake, they had seen some unusual sort of light in the sky or air, and some of them said that it appeared to come up out of the earth. At the moment, I looked upon this as a superstitious tale; the rather, as Padre Mancini had not remarked it; but I find almost every one I converse with in these districts, speaks of having seen some unusual halo-like light in the ​ sky, before and not long before, the shock. Many describe it at second hand, and these differ much in their statements as to time of its appearance, and give no intelligible account of its character; but many others say they saw it, and attempt to describe it as something like the light long after sunset, streaming up from the horizon at some one point, a sort of zodiacal light or phosphorescent diffused halo; whilst some point to one direction, some another, as its azimuth of apparition. Others—and amongst these were the gendarmes at Polla—say that the light seemed to them to emanate from the earth itself; and those that were in the dark gloomy lanes of the towns before the shock, say that some sort of luminosity lighted them upon their way.
Much, most of this, may be but the fancies of an imaginative and wonder-loving people; but in a country where commnnication is so bad, and news travels so slowly, it would be remarkable if so widely-diffused a notion, and one without any obvious popular basis of suggestion, should be devoid of all foundation in fact. I have therefore recorded it; and before dismissing the subject I may add, that I found the same story prevalent in the valley of Viggiano also, but lost all trace of it farther south, east, or north, while to the westward I heard the first of it at Auletta. It therefore has this remarkable attendant circumstance that, if fabulous, the fable was confined to an oval district around the most disturbed region. Conjectures would be useless as to its nature, but future observation directed to the point, may determine whether some sort of auroral light may emanate from the vast depths of rock formation, under the enormous tensions and compressions, that must ​precede the final crash and rupture that produces the shock, or whether volcanic action going on in the unseen depths below may give rise to powerful disturbances of electric equilibrium, and hence to the development of light; just as from volcanic mountains in eruption lightnings continually flash, from the huge volumes of steam and floating ashes above the crater.
Atena, which in Pliny's time gave its name to the whole valley (campi Atenati), is a town of extreme antiquity, its name indicating its Greek foundation. It is situated upon the east side of the great valley, and stands upon the crest, of a spur of absolutely bare rock, jutting from the lateral range, and having a small transverse valley or gorge to the north of it. The slope or angle of emergence of the rock, to the north of the town, from beneath the deep alluvium is about 30° with the horizon at a (Fig. 187); the bedding ill-defined but apparently with a general east and west strike and steep dip.
Don Vincenzio Jachetti, an inhabitant, an intelligent man, who had been appointed a Deputy Sotto Intendente since the earthquake, accompanied me over the town, which has suffered terribly; its streets, especially upon the north and south sides, are choked with rubbish of fallen buildings to a depth of 10 or 15 feet above the former level, and encumbered with fractured and entangled beams and joists.
The Photog. No. 189 (Coll. Roy. Soc.) shows the east end of the cathedral church, and various adjoining buildings, looking nearly westward. The general evidence of the wave-path with steep emergenee from the north, is observable in the thrown and shattered north and south walls, and in the wedge-like mass, projected from the S. E. quoin of ​the chancel, and the roof of it and of the south transept fallen inwards. The horizontal direction of wave-path deduced from the thrown quoin, and from the average of five sets of fissures in the church, was 165° 30’ E. of north, and the angle of emergence 47° 30’. The angle of emergence given by thrown quoins in three other buildings (dwelling-houses), of much worse masonry, however, and therefore capable of less exact determination, was only 39° 30’. Lower down upon the eastern slope of the town a large heavily built range of building, with front walls to the narrow street 31/2 feet in thickness, running nearly east and west, shows fissures of two inches wide, on the level of the first floor above ground, which indicate a wave-path not far from north and south, 177° 50′ E., the nearest approximation. One of these in the soffit of the arch, over a "portone" leading to a "vicinello" is shown in Sketch Fig. 190, and gave a path exactly north to south. A wall of old and little coherent masonry, has had six feet in height of its upper part thrown off, parting horizontally along, at 38 feet from the base. The mass of material has fallen at an average distance of 20 feet from the base, towards the south, and a point or two towards the west. The angle of emergence deduced from this (the horizontal velocity as at Polla) is between 45° and 50°, dependent upon the constant adopted for the adherence of the mortar and masonry at the base of separation. The result sufficiently corroborates those above given from more certain data.
The difference of effect of the same shock, upon well and ill-constructed buildings, is forcibly shown here. The square campanile of the church stands nearly isolated, its north and south side walls being nearly parallel with the axial ​line of the cathedral; the east and west walls stand 120° west of north. It is about 90 feet in height and 22 feet square at the base. The walls, 3 feet 8 inches thick at bottom, and only 12 inches at the summit (Photog. No. l881/2 and Fig. 180) are very well built, with large, long-bedded, heavy ashlar quoin stones, 3 to 4 feet bed along the face and 16 to 24 inches deep; cut limestone jamb linings and string courses; and the filling in between these, well-laid coursed rubble. At each of two points of its height—viz. the first and second string courses—the walls are counected by four slender chain bars of 11/2 in. $\times$ 3/4 in. iron, with transverse cotters outside the wall faces. This has stood uninjured, without even a crack, in the midst of surrounding ruin, a clear proof of what sound and good building would do, in securing the safety of the inhabitants of the towns, in earthquake countrie. High up upon the rocky hill side above the town also, are many summer lodges (scaffœ) which are very well built, and of recent date; and although probably a thousand feet above the town level, they have suffered very little: they are chiefly buildings of a single story, and owe their safety to this and to their good construction.
A large portion of the ancient walls of the town remain, probably of mediæval construction. At one part of these a large cylindrical tower existed, which for ages had been used as a cemetery. From the side of this, overhanging the precipitous face of the hill, a large mass had been thrown, and had exposed to view, the surface of a solid cylinder of human bones, of several feet in depth, those at the bottom reduced almost to crumbled bone-earth, while those on the
The position of the tower is imperfectly seen in Photog. No. 191 (Coll. Roy. Soc.), and its appearance in Sketch Fig. 192 (Coll. Roy. Soc.): the tower was about 28 feet in internal diameter.
The time of the first great shock was marked here by the stoppage of the communal clock at 10$^h$ 15$^m$ Italian time reduced to Frankish, but no exact reliance can be placed upon this. Signor Jachetti admits that all their clocks and watches about the country are either set by sundown or by the watches of travellers coming from Naples or elsewhere.
Upon the bank of the Calore, out in the centre of the plain opposite Atena, Jachetti pointed out to me, a fissure in the deep clay soil, which had been opened nearly parallel to the stream. It was simply a land-slip of a few hundred feet in length, the fissure 6 or 7 inches wide, and the vertical descent about the same, and originated at the violent shake at the shock.
Immediately behind the town, in the small lateral valley, ​a fissure also exists in the earth, which Signor Jachetti affirms when first opened extended into the rock beneath, but that the rains have since filled the latter in. I have much doubt of the fact, however, from his description of the appearance of the rock at the alleged fissure, which rather seems to have been an ill-defined junction of bedding at a steep angle, and that the outer bed of rock had slipped a little downward and outwards, over and from that on which it reposed. The earth fissure, however, was traceable for several hundred yards beyond the saddleback of a colline, connecting the spur of Atena with the main range of lower hills behind. It appeared originally to have been about 2 or 21/2 inches wide, and one side had descended about 3 inches below its original position. It was in earth over the limestone, varying from 2 to 4 feet in depth—a manifest case, like that in the plain, of slippage and shaking off of loose material, and not of actual fracture by bending or dislocation (Fig. 193).
A second small gorge, with deep and precipitous sides, runs in an east and west direction behind Atena, and falls ​of crumbly limestone rock have taken place, from both its faces, but chiefly from that on the south side, where the detached masses have fallen from the ends of nearly vertical ill-defined beds, whose strike is N. W. and S. E. This, like every case of fallen rock that I have so far observed, has been detached from a vertical or nearly vertical bed, where, owing either to the joints of the beds themselves or to cross fissures, there was little or no adherent connection with the adjacent rock; in fact, cases of loss of equilibrium and fall by inertia, and not of rending asunder through the solid stone, and dislocation by the direct energy of the shock.
Upon the opposite or west flank of the Vallone, and further south, stands Diano, the town from which it takes its name, upon a low, stumpy, jutting-out spur, of soft limestone, to the eastward of the great range (see Photog. page 165, No. 108, Part I).
The main direction of this spur, is nearly due north and south by compass, rising gradually from the plain at the north end; and it is completely cut off from contact with the great lateral chain of mountain, except at nearly the level of the plain, by the long lateral Vallone del Raccio, which brings in one of the great feeders to the Calore on its left bank, and whose bed in the bottom of the vallone, seems to lie in the line of a great dislocation.
This town has suffered comparatively little by the shock, many fissures, and a few of the old ill-built miserable class of houses thrown down, direction apparently north to south, none affording good data.
The beds of limestone rock at both sides of the valley, from above Atena southward to below Diano on the west, ​and La Sala on the cost, are vertical or nearly vertical (so far as bedding can be discerned at all), and the line of strike is nearly east and west.
The comparative immunity from destruction of Diano, is not difficult to explain. The direction of wave-path was here nearly due north to south. It therefore passed from the deep clays of the piano into the long spur or colline of the town, end on, (see Sketch Nos. 194, 195,) losing a large portion of
its vis vivâ at the junction, and a still larger portion, in passing through the great number of nearly vertical beds of limestone, about a mile in total thickness before reaching the town, in a direction perpendicular almost to their planes, like a bullet shot through the leaves of a thick book.
Again, the shock transmitted southwards through the lengthway of the great flanking chain to the westward, was almost completely cut of from reaching Diano at all, by the Vallone del Raccio to the north and N. W. of the town, upon ​the S. W. side of which, on the steep slope of Monte Mottola, the effects of the partial extinction of the wave at its surface as "a free or outlying stratum" were visible in considerable falls of projected rock (loose masses chiefly). Nothing of the wave passing along the flanking range reached the town, therefore, but secondary waves of refraction and dispersion, coming up from beneath the town, as the residue of the unextinguished original wave passed southwards.
Few better examples may be found, of the important effects of local condition, as modifying the effects of shock, or of the care necessary to observe and disentangle the phenomena. Of towns situated within three or four miles of each other, one is found almost totally destroyed, the other is scarcely injured. It seems inexplicable at first sight, that both should have been almost equally near, to the same subverting agency from beneath; yet nothing is simpler or more certain when explained, than the conditions which shielded the one, and left the other exposed to destruction.
The protecting circumstances as respects Diano will be understood by comparing Sketch No. 194 with the Section Sketch No. 195, supposing the line a b to be that of the wave-path.
St. Arsenio, Torre, and St. Pietro, small places on the west of the Vallone Diano, but north of Diano town, were also more or less protected by similar conditions; cut off from the great flank range, by the little lateral valley of the Aqua del Secchio, and others. They suffered much more than Diano, however, and St. Rufo, on the south flank of the lateral Valley del Torno, still more than either. The ​wave-path at St. Arsenio and St. Pietro was, from fissures, 142° to 144° W. of north.
Much further away to the westward, (10 to 15 miles west of Diano,) in the heart of the mountains, and in the great extent of rugged country, south of the very high table land of Piano di Salvagnuola, and still more south, in the valleys of the rivers Carmignano, Calore, (another west of the river of the Vallone Diano,) Pietra, and Cilnio, the earth-wave must have been propagated with much violence, but with frequent and rapid changes of direction, and hence rapid loss of vis vivâ and speedy extinction. Castelluccia, Ottati, Corbeto, Laurino, and some other towns were greatly damaged. But there are vast tracts of uninhabited mountain and valley about here, and little is known of the shock in these, which I was unable to enter, as they were all under snow of considerable depth.
Still further west, however, after the confluence of the above-named rivers which divide the country for above twenty-five miles in a nearly north and south direction (by compass), though in an irregular line, and previous to the approach of this river to the Bosco Persano, before its junction with the Salaris, the long valley of the western Calore arrested almost completely the violence of the shock, so that between it and the Sea at Pæstum little of it was experienced.
Returning to the Valley of Diano, upon a new piece of the military road not yet used, between Atena and La Sala, was a newly erected culvert, of three semicircular arches of 12 feet span, passing a torrent under the road, the piers, about 8 feet to the springing, all built of good squared ashlar, the arches turned in brick, two bricks thick. ​The structure was not overloaded with material and was well put together, and the mortar still green. It did not exhibit a trace of injury. (See Fig. 196.)
As I pass southwards, still in the valley, and approach La Sala, the mountain peaks to the rear and above the first low range of the east flank, all show evidence of increased looseness and softness of the limestone rock; its bedding becomes more and more indefinite, its minute structure, more and more like a mass of fine angular compacted fragments, of a rather harder and originally more uniform liassic-looking rock, and its lithological character one of increasing chalky whiteness, with more and more silex intermixed, in the state of a very fine gritty white sand.
The forms of all the mountains behind, the rounded culmination of their summits, the curves of their flanks, with the flowing lines of the ravines, and swelling protuberances of the hill-sides, all alike indicate, a very soft and easily denuded or weathered rock; one of low elasticity and density, and capable of transmitting impulse, much less powerfully, and to a much less distance, than the limestone I had already encountered, and which with its maximum ​hardness and sonoricity I had found in the flanking peaks of the Valley of the Tanagro, some twenty miles to the north.
Many isolated houses and other buildings about here, founded upon the deep clays of the piano, exhibit by their fissures, an almost completely uniform direction of wavepath north to south, and an angle of emergence so small as to seem almost zero. I observe, however, that wherever such buildings are founded upon the limestone rock, upon the gentle slopes, of the lowest hill sides of the east side, of the Vallone, the wave-path tends a little to the E. of north, i. e. it seems to come from the line of the eastern flank range, more or less, but still with the prevailing north to south path; the divergence towards a N. E. to S. W. direction being from 10° to 25°: and the angle of emergence, at once changes from nearly zero, to a pretty large one, but which gradually decreases as I travel south.
Within a mile of La Sala, on the left of the road, stood a square, strong-built house, which was perfectly cardinal, and afforded excellent measurements by fissures and thrown wedges. It gave a subnormal wave-path nearly 171° W. of north, and an angle of emergence of 24°. Another cardinal building to the right of the road just entering La Sala, shows a subabnormal wave-path 155° W. of north, and with nearly the preceding angle of emergence, the wave here again appearing to come from the line of the lateral range to the eastward.
Nearly opposite the town, I observe at a quarter of a mile out in the plain, several large haystacks (Fig. No. 197, Sketches Coll. Roy. Soc.) leaning over at top very much to the southward. They had their longer axes nearly ​east and west, and all were thrown to the south, without any twist. The people about, said they had been all built plumb, and were so, before the shock, an interesting proof, that a very light body may be overturned by shock, equally with one of great density, the inertia of motion being exactly proportionate to the weight.
La Sala.—This town, although of Roman, if not of still earlier origin, and showing remains of much antiquity about the old Castello, seen above the town, is nearly all now of modern building; and upon the whole far better built, than any town I have yet seen in the Vallone. It extends for nearly a mile and a quarter, along the slope of the hill-side, the buildings rising above each other, and presenting very generally their greatest length in north and south directions, or parallel to the hill-side, which is nearly continuous, and unbroken by any deep lateral gorges, all along the east side of the Vallone. Not a single stream of any magnitude falls into the Calore from this, but all its feeders from the other or western side. This town is the seat of government of the province, and contains many large official and other structures. All are more or less fissured, but the actually demolished buildings are few. This seems to have arisen less from diminished energy of the shock here, than from the substantial character of the buildings, and from the fact, that almost all of them presented their long dimensions to the line of shock, as may be seen in Photog. No. 198 (Coll. Roy. Soc.) of the town locking from the N. W. The general position of the buildings in plan, as they wound along the hill-side, is along a curve, as in Fig. 199 (Sketch, Coll. Roy. Soc.). There is an elevated little valley, at the back or ​east of the town, between it and the ridge of the Costa della Madonna, of arid limestone, and slender covering of soil at various points, but no fissures or falls of rock were visible.
I found the Sotto Intendente, Il. Cavalieri Gul°. Calvoso, living with the Signora in a comfortable wooden "barrac" or hut beneath the town; for although the great shock threw down so few buildings here, the alarm of subsequent minor ones, has caused those who could, for the present to desert their permanent stone houses. He accompanied me with his secretary, Il. Caval. Ferdinando Lansalone, through the town, and through his own palazzo (the Casa Officiale), which, though shaken and fissured, was still standing, just as it had been fled from by every living being, on the night of the 16th December; and as it had been locked up ever since, the pictures and many other objects within the house, were lying strewed or thrown about, exactly in the positions in which the shock had left them. The Sotto Intendente gave me on the spot, and in the rooms, a very graphic and intelligent account of his observations as to what had occurred.
There is no record as to the precise time of the shock. The clock at the Casa Communale was thrown down and stopped, but the hour could not be got from it, and its inaccuracy was admitted to be as great as usual.
The house of the Sotto Intendente is founded on the solid limestone rock: it is a long and rather narrow two-story building of large size, stone built, with timber and filed floors and roof, and well constructed. There are a few small fissures in the walls, indicating a north to south wave-path, emergent 20° to 30°, but the latter evidence is uncertain. ​The long axis of the house, has a direction 20° W. of north. The room occupied by the Sotto Intendente with his family on the 16th December, is a nearly square one, on the first floor (i.e., one over the ground floor). They had not gone to rest, and he was first alarmed by a short, sharp rattling, with a jumping vertical movement of about half an inch, of a large white metal chocolatière, that stood upon a marble-topped table at a, touching both the south and west walls. At the same instant he heard the "Rombo," which continued during the entire time of the shock. It was not very loud, but very terrible, and "seemed to make the floor and the whole house to tremble"—a hoarse and grating rumble. Before he could have reckoned twenty he thought, the great shock came, a distinct undulation, which several times swayed everything back and forwards, and lifted up and dropped down simultaneously, the horizontal movement having by much the greater range.
As far as he could judge by his own perceptions, the range of horizontal motion did not exceed half a palm (3 or 4 inches). The movements did not instantly cease, after these great oscillations, the total number of which he could not be certain of—he thought they did not exceed four or six—but all was quiet after (as he supposed) about half a minute, when they all rushed out of the house. He never himself, lost his presence of mind; on the contrary, he said that the minutest circumstances of movement, &c., that occurred in the room, from the instant when the chocolatière began to give tongue, seemed to stereotype themselves, upon his observation and memory.
A number of glazed lithographs, in fiat wood frames, each hung from a single nail, upon the north and east walls of ​the room, $\mathrm {A}$ and $\mathrm {B}$ (Fig. 200). Within a second or two after the chocolatière had begun to jump and make a noise, the lithographs hanging upon the wall $\mathrm {A}$, began to oscillate slightly in the plane of the wall, or from east to west, and the reverse; and at the same instant, those hanging upon the wall $\mathrm {B}$, began to oscillate slightly, out from and back to that wall, i.e., in the same east to west direction. The great shock now arrived, and the frames upon the wall $\mathrm {B}$, at once began to sway forward and back in the plane of the wall, or in a direction south to north, and the reverse; while those upon the wall $\mathrm {A}$, commenced the movement out from and back to the wall; and for a moment or two he thought they all moved more or less both ways, viz., in the planes and at right angles to the planes, of both the east and west and north and south walls. The motion ended finally, by the prints on the wall $\mathrm {B}$, alone oscillating gently in its plane, with a decreasing motion, for two or three seconds, and finally coming to rest.
Of the lithographs upon the wall $\mathrm {A}$, the Sotto Intendente pointed out to me one, the dimensions of which (they were all quite similar) are given in Fig. 201, and he caused it to vibrate in both ways by his hand, as nearly as he could to the me extent, that he had observed it to have moved at the most violent period of the shock. The chord of the arc of vibration, in plane of the wall (east and west) was about 2 inches, and the semichord of the corresponding vibration, from and back to the wall $\mathrm {B}$, was about 1.25 ​inch. The chord of vibration of the frames on the wall $\mathrm {B}$ in plane of that wall (or north and south), was 7.50 inches,
and the semichord corresponding from and back to this wall $\mathrm {A}$, was about 4 inches.
It was obvious, therefore, that here two wave-paths almost coincident in time had crossed each other, at a sharp angle, the one arriving first, being transmitted with more or less of an east and west direction, from the north and south axis of the great range of mountains to the eastward, and having an horizontal amplitude not exceeding 2 inches; the other, which almost instantly followed, having a north to south direction, and an horizontal amplitude of 6 or 7 inches.
Nothing observed, except the chocolatière, gave any approximation to the extent of vertical movement or altitude of the wave which appeared by it, about half an inch at most.
Upon rushing out of the house, Signor Calvoso said, he was struck with the quietude of the night, and the general serenity of the sky for the season. The night was not a dark one, but he had observed nothing himself, of any unusual luminous appearance; of having remarked which, however, numbers of persons had spoken to him since the shock. He however stated that having on the ​instant, to attend to many official calls, as well as domestic ones, incident to the alarm of all around, he might not have remarked any such phenomena, outside the house.
In his "salone," or drawing-room, a large clumsy pendule in an irregular hexagonal frame, with the dial of about 10 inches diameter, in the centre, like a picture, hung upon the wall parallel with $\mathrm {B}$, in last Figure (i. e., in a plane 20° W. of north), by a single nail and ring at top. A nail driven into the wall at N. (Fig. 202) was in contact with the frame, when the whole hung plumb, and prevented all movement of oscillation towards the S., but it was free to oscillate in the opposite direction, the nail corresponding to N. having been withdrawn from some cause. I found this pendule remaining out of plumb, and thrown to the northward, as shown by the dotted line (Fig. 202), so that the edge of the frame had moved 1.75 inch, from the nail at N. The centre of gravity of the whole, I found by trial, was in the centre of vertical figure, and the weight of the whole was 81/2 rotuli = 16.49 lbs., the frame projecting 51/2 inches from face of wall. By trial I also found, that the friction of the back against the wall, required a force of 11/2 rotuli = 2.91 lbs. to set it in motion from rest. Had it been free of the nail at N., it of course would have vibrated through an are of 3.50 inches, assuming each semiphase of the ware to have equal velocity, and neglecting the effect of emergence.
From the considerable weight of the pendulum and the large proportion the friction bears to the weight (nearly 1:5), it forms by its range of motion an approximate measure of the amplitude of the wave here in an horizontal direction one too inexact, it is true, to found any ​calculation upon, but yet enough to convey a distinct notion of the extent of movement of a powerful earthquake shock.
The actual lateral movement here was probably about 31/2 to 4 inches—a range of motion which, made with a velocity as great, as that with which one reaches the ground, on leaping down from a height of 21/2 feet, may enable one easily to understand how readily persons are thrown down when exposed to it.
The Secretary Lansalone pointed out to me in his house two brass table lamps, fashioned as in Fig. 203, which stood upon a semicircular table, placed with its diametral side, in contact with a wall running 37° W. of north, the lamps being placed so that the line $xy$ was at right angles to the plane of the wall. The centre of gravity of either lamp, is 7 inches above the base, and the centre of oscillation is about 10 inches above the edge of the base, considered as centre of motion. The weight by trial = 1 rotulo 2 unci.
At the shock one of these lamps was thrown off the table towards the north and the other towards the south, and lay upon the floor in the positions shown in Photog. No. 205, (Coll. Roy. Soc.), and Fig. 204. The china things upon the same table remained as seen by me, nearly undisturbed: they were low and broad based.
The legs of the table were somewhat elastic, and as they sprung under the shove from the wall, in contact with it, no deduction as to wave velocity can be made from these lamps. Making some allowance for the effects of this elasticity, in giving divergence to the direction of throw, ​normal to the face of the wall, the position of the lamps indicates a wave-path, about 157° W. of north.
There are many pictures in this house in rooms unused, hanging from single nails, which have been caused to swing in the plane of their respective walls. Many remain, just as left by the shock, and all tend to show a general north to south direction, with more or less of movement from a diagonal line approaching east to west. Their friction was too great against the rough walls, to admit of further deduction.
The Sotto Intendente, who, though a keen observer and very intelligent, knows nothing of science or of earthquake speculations, remarks to me that he has observed the buildings situated on the harder limestone, everywhere in his province, much more shaken and injured, than those posited upon the softer chalky stuff found here and further south.
$cc$
Fig. 206.
The appearance of the church and tower is shown in Photog. No. 207 (Coll. Roy. Soc.), as seen from the N. W. Nothing can be inferred from the fall of this bell as to the direction of the wave-path (which we have already got from other data), inasmuch as it was obvious that a line drawn from the centre of the tower to the centre of gravity of the bell, at its place upon the ground at $g$, would be far from representing truly the plane in which its trajectory of descent had been made. It had first descended to $n$, and coming in contact with the eave tiles, had then taken a new course, and been thrown directly outwards or westward from the wall, in its further descent.
It was also observable from the scratches, &c., on the bell, that the eastern pintle had given way first, so that the full force of the shock had not acted in projecting it. The shock acting at the centre of gravity $s$ (No. 2), and the resistance of the last held pintle $p$, had caused the bell to rotate slightly before falling, and given it along with its glancing off the eave tiles, a much more westerly direction of descent, than was due to the direction of wave-path alone. Both these extraneous forces also had reduced the horizontal distance to which it would have been thrown, had it freely pitched to the ground in its original path—both pintles being freed together.
Taking the vertical height of fall to be that from the centre of gravity of the bell as it hung, to the level of the eave tiles, and the breach therein as marking the horizontal distance thrown, and applying the equation—
${\text{Tan}}\,e={2H+{\sqrt {4H(H+b)-a^{2}}} \over a}$
$b=26$
$a=16$
$a$
$a=17$
Taking the velocity of the wave as we found it at Polla $=13$ feet per second,
$\mathrm {H} ={\frac {v^{2}}{2g}}=2.62$, the height due to $\mathrm {V}$,
and solving for $e$ — we have
$\mathrm {Tan\,} e={\frac {5.24+{\sqrt {299.9-289}}}{17}}=0.513$
and $e=27^{\circ }10'$
which is the angle of emergence, as given by the projection of this bell. This, owing to the disturbing conditions, can only be considered as approximatively true, to within ± or 3°. The result, however, is corroborated by the same angle having been obtained otherwise, as already given: better data were not procurable at La Sala.
The base of the church tower may be viewed as about the mean level of La Sala. The barom. reads 27.86 in. Thermo. 54° at 11$^h$ 0′. A.M. Naples time, 15th February, which reduced gives the altitude of the spot = 1768.30 feet above the sea, and about 1000 feet above the piano at its northern extremity.
The cause of the disturbances in the general direction of wave-path observable at various points since leaving the north end of the valley, began now to be apparent, on examining with the maps, the relations of the lateral chains at either side, to the main direction of shock hereabouts.
Referring to Fig. 208, in which is sketched the general form of the piano, and the prevailing lines of ridge, or summits of the mountain ranges around it are marked by ​dark lines; the main direction of the wave-path from Campostrina southwards is indicated by the large arrows w, w, and is that given generally by everything in the
piano, south of Polla, to Padula. But on the east side of the valley, going southward, it is constantly disturbed by the intermixture of another wave-path oblique to the former, and with increasing obliquity the further we go south; this intersecting angle, being small at Atena, greater at La Sala, and greater still at Padula, and the direction of wave transit of this secondary shock being always from N. E. to S. W., or from the lateral chain to the east of the valley.
It is obvious that the impulse given at the north end of the valley in a north to south direction, simultaneously to the deep clays of the piano, and to the eastern mountain range at its north end, was transmitted comparatively undisturbed as to direction through the former; but, in passing along the lateral ridge of limestone mountain across all the nearly vertical beds, transverse vibration was produced along ​the whole line of mountain crest, which oscillated transversely, like a thin plate or musical string, struck at one point, giving rise to a secondary wave whose path was that of the wavy line s, s, s; the transverse or east and west excursions becoming greater the further the originating wave travelled south, and being both exaggerated and disturbed in direction, by the many curves and sinuosities in the mountain axis not shown in the sketch. Every point situated upon the limestone rock, therefore, at the east side of the valley was exposed to two shocks; the primary in the direction nearly north to south, and the secondary (from the transversal vibration of the mountain chain) more or less oblique to that; and these arrived, not quite simultaneously, but with an interval of time, as well as an angle of intersection, greater as the point lay further south. Such traversal vibration, of the almost continuous though sinuous ridge to the east of the valley, was of course not confined to the horizontal. The chocolatière of Sig. Calvoso, at La Sala, gave evidence of the same in the vertical, and also shows that the originating wave traversed faster through the limestone rock, notwithstanding its loss of vis vivâ in penetrating across from bed to bed of east and west stratification for so many miles, than through the deep clay of the Vallone. An examination of the sketch will also indicate the reason why the towns at the west side of the Vallone received so much less damage than those on the east side. All the feeders of the Calore fall in upon the west side, not a single one upon the east; the west lateral range is therefore cut across by lateral valleys, down nearly to the level of the piano, while at the opposite side the range is continuous. The wave therefore ​was either diverted westward, or nearly extinguished at the western side of the valley, before it had reached far to the south, while at the eastern side it was carried on by the continuous lateral chain until changed in direction, and greatly diminished (by the free stratum) where the continuity of this range is broken and terminates, near Padula.
About a mile south of La Sala, near the military road, and standing at the edge of the piano upon clays of a few feet in depth, overlying the limestone lowest roots of the hills where they dip beneath the plain, stood the Chiesa della Trinita; the short tower, the roof and west end of which have all fallen, and the walls are fissured severely. It is seen in Photog. No. 209 (Coll. Roy. Soc.) looking westward. The tiled roof had fallen within the walls, leaving the tiled eaves still upon the tops of the side walls: its debris had been removed, but I was informed that the larger portion was found packed against the north flank wall upon the floor. The west end was thrown to the S. W. The east end had been kept up by cross walls external to it. It presented fissures, best seen at the east side of the wall (not shown in Photog.) which showed an emergent angle of 23°. The wave-path could only be approximated, but was a few degrees W. of north to south.
The Ponte Silla, an ancient Roman bridge over the Calore about a mile and a quarter south-west of this, founded probably on piles, with its heavy piers deep in the clays of the valley, has suffered no apparent injury by the shock.