WEIGHING MACHINES. In this our concluding article on weighing instruments, we intend to supply such particulars as have not already been treated of under the heads of Balance and Mechanics. In doing so, we propose, in the first instance, to sketch shortly, and possibly with some novelty, the early history of the invention, which claims attention from the high antiquity it holds among the mechanical arts; and then to furnish descriptive notices of several balances, some of them of recent date, and not described, it is believed, or at least collectively so, in any previous work.
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The most ancient instrument used for ascertaining weight, and the one from which the others were derived, was probably the common or equal-armed balance. It may have been invented much about the same period as the yoke or swing-tree employed to attach two beasts of burden to a common load. Both inventions were nearly allied in construction; and their approximation in the nature of the object to be effected by their instrumentality, leaves little doubt that the one originated the idea of the other. In their first rude formation, they seem to have been indiscriminately used, both for weighing and equably dividing the labour of draft. Accordingly, the Greek words ζυγὸς and ζυγὸν, and their Latin derivative jugum, were employed to signify either the balance or yoke. In the Hebrew, the word used in the oldest books of Scripture to denote the balance, is one which is descriptive of the instrument itself, by having reference, through related words, to weighing and the ascertainment of weight; while the word for yoke, without having descriptive reference to any instrument, signifies literally upon; seemingly in allusion to an already known contrivance being laid as a yoke, over or upon the necks of the two draft cattle; a custom which still prevails in the east, as it anciently did in all countries. From the analogy furnished by the double application of the Greek and Latin terms, it may be concluded, that the beam of the balance was the instrument thus apparently referred to, and that it was therefore the earlier invention of the two.^[1. Archdeacon Woodhouse (on the Apocalypse, 1805, p. 144), in explaining some peculiar views in regard to the translation of the Greek word ζυγὸν, gives priority to the yoke, rather incidentally perhaps than otherwise, since the question of date does not affect his argument. ] '

That the balance was the first of strictly mechanical contrivances, is not perhaps improbable. Its great antiquity is attested by its being the only machine commemorated among the twelve signs. The name bestowed by the Greeks and Romans on this girdle of constellations was ζωδιαχὸς, zodiac, a word having reference exclusively to living creatures. The balance being an inanimate object, seems therefore to have been long rejected by them, and its place supplied by giving to Scorpio a double share of the heavens; the twelve names being then completed by applying the word Chela, to denote that half of the sign into which the two claws of the animal extended. The introduction of Libra, or the balance, does not seem to have been fully sanctioned even in Virgil’s time; for although elsewhere named by him, he describes its place as offering a favourable vacancy for the reception of Caesar among the constellations; and poetically figures the scorpion’s claws as already contracting to make room for him.^[2. Georgics, i. 32. ] The ancient claims of the balance were however revived, and the name of Chela disappeared from the number of the signs. Among other nations, the balance held its place in the zodiac from the highest antiquity. Along with the other signs, it is sculptured on the most ancient monuments of Egypt; but that this was not their birth-place, may be inferred from the contrast between the figures of the signs and those native and strangely fantastical devices with which the Egyptian monuments are otherwise so profusely covered. The Abbé Pluche, arguing from another circumstance, viz. the non-accordance of the seasons in Egypt with those which he considers the signs as marking, traces their invention to the plains of Shinar, and refers it to the near descendants of Noah.^[3. Pluche, Histoire du Ciel, Paris, 1739, v. i. pp. 18—30. A short memoir of this author will be found in its proper place in a former volume of the present work. ] The annexed figure represents an ancient and well-formed balance from Egyptian sculpture. For the sake of distinctness, however, it is here shown as off the equilibrium. A line and plummet hang from the beam, to mark when a tongue, which points downward, reaches the vertical position.

Others, of a much ruder description, and without any contrivance to indicate equilibrium, are given by Rosellini, from ancient Egyptian paintings.

The steelyard has generally been regarded as of greater antiquity than the common balance; but in point of conception it is less simple, and likely therefore to have been of subsequent invention. The correctness of this inference admits perhaps of being fully established. In Job and the Pentateuch, by much the oldest records extant, the Hebrew term employed to denote the balance is moznaim, a word of plural, or, as pointed by the Masorites, dual import. The kind of balance intended by it can be no other therefore than the common one; the dual word moznaim evidently bearing reference to the two scales.^[4. In nine passages of our version of the Bible, mornaim is translated in the plural balances, and in seven passages balance, Were the term scales employed for the plural rendering, and pair of scales for the singular, the English would express the original with much fidelity. ] That it cannot mean the steelyard, appears to be clear, because in it only one scale was required, while in every respect it is destitute of any double member from which a dual name could have arisen. This view, which appears to be decisive, is confirmed by the definition given of the word by Gesenius, who explains it to mean the balance, properly a pair of scales. ^[5. Leo’s Gesenius, Cambridge, 1825, p. 386. The argument (in support of which Gesenius is here quoted) receives similar confirmation from the Greek, in which the singulars ζυγὸς and ζυγὸν. were generally employed to denote the yoke, and their plurals the balance. ]

The common balance being therefore the one mentioned in Job, is ascertained to have been in use 2130 years b c . or 218 years after the deluge.^[6. The chronology here followed is the common one, adopted and specially vindicated in this instance by Townsend (Chronological Arrangement of the Old Testament, 1826, v. i. p, 28). Dr Andrew, in his Scripture Chronology, 1822, p. 33, dates the time of Job 2215 years b c . and Dr Hales, in his Analysis, vol. i. p. 185, 2337 b c . ] How much earlier than this it may have been used, cannot safely be conjectured. The knowledge of it probably came down, through Noah’s family, from antediluvian times; for without some means of ascertaining weight, the ordinary transactions of every-day traffic could not in any age have been carried on.

From the preceding observations, it appears likely that any allusion to the steelyard in the Bible should be made by a noun possessing a strictly singular termination. The earliest use of such a word having reference to the subject, is in the time of Solomon, about 1014 years b. c. It occurs in Proverbs x xvi. 11, where the term pheles (explained by Gesenius, p. 643, to be a balance, equipoise, properly a steelyard) is followed by the word moznaim, already noticed. In the English version, this passage is translated a just weight ^[7. Among the Hebrews, as among the Greeks, the shekel, manch, and talent, were names of particular weights, by which their money and other precious articles were weighed. The Hebrew word for commercial or general weight was eben, literally a stone, and ebenim, in the plural, stones; probably the origin of the term stone weight. ] and balance, &c. A more correct rendering would be, a just balance and scales; first, because pheles has been preserved in the Arabic to the present day as meaning, not a weight, but a weighing instrument; and, secondly, because the word occurs again in Scripture, viz. Isaiah, xl. 12, where its sense is fixed by the immediate [7:21:836]context. In this passage, as in the former instance, the word pheles precedes moznaim, both words being brought together, and occurring in a parallelism. The passage referred to is rendered in our version, “who hath weighed the mountains in scales, and the hills in a balance.” The word here rendered scales in the plural, is the singular noun pheles; while that rendered balance, is the dual word moznaim. The English therefore would better accord with the original, were the terms in the translation reversed in order, and read thus: “who hath weighed the mountains in a balance, and the hills in scales.”

Whether pheles, however, means properly a steelyard, as Gesenius defines it, may be doubted. The reason for supposing it does, seems apparently to rest on the circumstance that kapheles in the Arabic (most probably a corruption of hapheles, the definite form of the Hebrew word) is the name of the Arabian weighing instrument of the present day; and because it now is a steelyard, and is known to have been in long and almost exclusive use in Arabia, and indeed in all Asia, that therefore the related or parent word pheles must denote the steelyard. But this rests on the assumption that the common balance had never been known to the Arabians. Job, however, who is shewn to have been an Idumaean Arab, and to have resided in the north of Arabia,^[8. Horne’s Critical Stud, of the Scriptures, 1825, v. iv. pp. 74, 75. ] does not use pheles, but moznaim, ^[9. Job, vi. 2, and xxxi. 6. ] to express the balance; a word which, as already shewn, denotes the common or equal-armed one, and at that period of course undoubtedly used in Arabia.

Perhaps both words may therefore be regarded as synonymous terms for the common balance; the one descriptive of it by reference to its beam or balance-rod, and the other by reference to its pair of scales. This view receives further confirmation from the circumstance that there is one Hebrew word in the Bible which may be shewn to refer with some distinctness to the steelyard. Isaiah, in a passage subsequent to that above quoted, when describing the construction of idol images, says, “they (the Babylonians) weigh silver in the balance,” &c. (chap, xlvi. 6); and here makes use of the Hebrew word kaneh to denote the kind of balance meant. Gesenius, p. 699, explains this word to be a cane, or measuring reed, or scale for measure; a beam or lever of a balance, or balance itself The word has thus evident reference to a graduated staff or beam,^[10. In this sense it occurs in many passages. Thus, Ezekiel, xl. 3-5: " With a line of flax in his hand, and a measuring reed...and in the man’s hand a measuring reed of six cubits long; so be measured the breadth of the building,” &c. ] and probably therefore when used, as in the present instance, to express a weighing instrument, denotes rather the graduated steelyard, of which it is thus descriptive, than an ordinary balance or pair of scales. As it is not used to express a weighing instrument in any other passage of the Bible, and as Isaiah is here giving a minutely graphic picture of the process observed by the Babylonians in the manufacture and correct adjustment of the weight of their gods, it is probable that the instrument named was an eastern invention, constructed in all likelihood of the sugar or “sweet cane,” a sense in which the word kaneh is also used.^[11. Thus, in Jeremiah, vi. 20: “To what purpose cometh there to me incense from Sheba, and the sweet cane from a far country ; your burnt-offerings are not acceptable, nor your sacrifices sweet unto me.” ] These remarks appear to receive some illustration from the fact, that in China and the East Indies a small steelyard (described in our article Balance, p. 306) is employed to weigh precious metals, gems, &c.; and to this day is made of wood or ivory, and not improbably also of cane, a substance which, from its combined lightness and strength, is well suited for the purpose.

This is probably the first mention of the steelyard on record, viz. in Isaiah’s time, or about 780 years b. c. ; at which f>eriod it thus appears to have been in use among the Baby-onians. It seems to have superseded the common balance in Asia; and, through the Romans, to have come into extensive use in Europe. The annexed figure is a representation of one made in the first century, and not long since recovered from the ruins of Pompeii, which town, with many of its inhabitants and their utensils, lay buried, for nearly eighteen centuries, in the shower of volcanic matter thrown out by Vesuvius, a. D. 79. Like some of our steelyards, this ancient one is provided with two different centres of suspension. It wants however the knife-edged bearers now universally used. In the progress of invention, the modifications of the steelyard have become numerous in the form of compound machines; but these are of comparatively modern date, and do not require successive detail. Descriptions of the most useful varieties of the balance will be found here, and under our previous articles on the subject; and as it is intended to embrace the present opportunity of classifying their different kinds under distinct heads, references to those already described will be introduced in their proper place.

I. Balances acting as Levers round one or more Fulcra, and requiring experimental Adjustment when used to ascertain the Weight of a Substance.

To this class belong the Common, the Roman, the Chinese or Indian, the Danish or Swedish, already described under Balance and Mechanics, pp. 358 and 370; and Magellan’s, Mr Bate’s, and Dr Black’s, under MecHANIcs, pp. 371 and 456.

This is a compound steelyard, and is used for ascertaining the weight of loaded carts. The apparatus, of which a ground-plan is here given, is arranged in a box ABCD, which is about twelve inches deep, and is sufficiently sunk in the ground to place the highest part of the completed machine on a level with the line of road.

A fixed wooden bridge, by which the horse may pass across the machine, is laid over the middle part of the box. At each side of this bridge is a metal wheel-track. On these the cart is rested by its two wheels and a prop-staff. The tracks are fixed to and borne up by a platform (or skeleton frame) placed below the wooden bridge, but above the works. The platform has four short feet, the lower ends of which are formed into small hemispherical cavities. These cavities rest at E, F, G, H, on pointed studs about an inch long. The two studs at E and F are fixed on the crooked lever AKB, with their points upward, and the two at G and H are similarly fixed on the companion-lever DIC. Sharpened studs, also about an inch long, project downward from the four extremities of the crooked levers. The points of these studs rest in small hemispherical cavities, formed at the upper ends of blocks which are fixed as bearers for them in each corner of the box. The short parts of the crooked levers parallel to OL, and near the middle of the box, have their under edges sharpened, and rest on the angle of a sustaining fulcrum IK, which passes through, and is fixed to the lever OM. This lever is also crossed at L by the fulcrum which forms the common centre of movement for the whole machine. The angle of the fulcrum points downward, and rests, as in the ordinary [7:21:837]balance, on concave arcs, situated one on each side of the lever OM. These arcs are formed on the upper end of a block, which rises from and is fixed to the bottom of the box. The lever is again crossed at M, by a fulcrum having its angle pointing upward; and from this angle a receiving scale for the weights is hung by a shears or double ring. In cases where it is wished that the machine when unloaded should preserve equilibrium, the lever OM is prolonged to the left, and loaded at that end, to the extent requisite to balance the weight of the levers.

A side view of BK, one of the limbs of the crooked lever AKB is here given, in the direction of the diagonal BD. The lever, as shewn in the figure, is kneed to an extent sufficient to admit of the angular points of the fulcra being placed in the same horizontal line. The other limb AK is of the same shape. The form of the entire lever AKB is thus easily understood. Its companion lever DIC is of the same construction. The limbs of the levers are therefore so shaped that the fourteen sharp bearing parts of all the fulcra are situated, during equilibrium, in one horizontal plane. These bearing parts, and the parts they bear against, are all formed of polished steel.

If now EA be supposed a tenth part of the length of EK, then 100 pounds placed on the platform, or on the wheel-tracks, and pressing therefore on the studs E, F, G, H, will be sustained by ten pounds of an upward pressure at IK. And if, again, OL be a tenth part of LM, the ten pounds of upward pressure will be given by one pound placed in the receiving scale at Μ. These proportions are assumed, for easy arithmetical illustration; but in the practical construction of the machine, the distance between the resting points at E, A, and at O, L, is made to bear a greater ratio to the distance between the resting points at E, K, and at L, M, than has here been supposed. Instead of using a receiving scale, the lever OM may be prolonged to the right, and graduated as a steelyard. And in machines where an upward instead of a downward pressure is given at M, the lever OM is prolonged to the left, and the fulcrum L and its resting-block removed to N.

Machines consisting of variously modified systems of levers have been used for a considerable period. The one above described is that generally employed. The arrangement of the levers is well conceived, and exactly suited for the purpose to which the machine is now chiefly applied. Its first contrivance and employment is said to have been for weighing the riders of race-horses.

The beam of this balance consists of two rods, at some distance apart, but united so as to move together. The scales are supported above this rectangular beam; the rising and sinking of the frames in which they rest, and by which they bear on the knife-edges at each extremity of the beam, being regulated by a parallel movement, the levers of which are placed in the stand of the balance. It is understood to have been of French invention, and is a convenient form of machine; but the bearings, which are numerous, and liable to be somewhat displaced, give rise to considerable friction, and occasionally to slight derangements of the leverage. Some improvements on it in these respects were patented by Mr Poupard of London. In his machine there are still however three parallel knife-edge bearings, three circular ones, and other sources of friction, which, from the construction necessary for the proper ascent and descent of the scales, cannot be avoided. The working parts of the machine are mostly cased up to protect from dust. In the annexed figure, some of these casings are omitted, that the representation may be properly intelligible.

II. Self-adjusting Balances, which derive their Power of progressive Resistance from Properties strictly mechanical.

Under this head we place Ludlam’s balance, introduced about 1760, and an improved modification of it, named the bent lever. Ludlam’s is described under Mechanics, p. 371, and the bent lever under the article Balance.

This balance, by Mr Brady of London, is provided with different points of suspension for the scale, and is a modification of the bent lever one. In the upper end of its lever are three cavities, at different distances from the centre of movement, from any of which the scale may be hung. For each cavity there is a separately graduated arc. The three arcs are placed one above another, and move through an opening in the bar or shears which carries the receiving scale, the edge of the bar serving the purpose of a pointer. When the scale is hung from the cavity next the upper end of the lever, the beam becomes horizontal, each ounce, as far as two pounds, being indicated on the upmost arc. When hung from the middle cavity, each alternate ounce, as far as eleven pounds, is indicated on the middle arc; and when hung from the remaining cavity, each quarter pound, as far as thirty pounds, is indicated on the lowest arc. In point of principle it differs from the bent lever thus far, that the acting mass of the counterpoise is altered (for the most part increased) by the accumulating weight of the graduated arcs and their strengthening stays as they pass the vertical line from the centre.

The balance by Mr Dampier of Ware is in this respect the reverse of Mr Brady’s. Here a circular disc or wheel is employed, which, as it turns on its centre, is balanced in any position, and therefore neither adds to nor diminishes the acting mass of the counterpoise, which is hinged on a round stud placed in the rim of the disc. The scale is similarly attached to the rim of the disc, a little more than a quarter circumference intervening between the two studs. The pointer is formed in the suspending frame or stand which supports the axis, and the graduations are marked on the upper part of the circle. This construction is a modification both of the bent lever and Ludlam’s balance, and, in cases where the weights to be examined are of small amount, is preferable to the former.

The foregoing self-adjusting balances are placed together, as being nearly allied, and having a similar extent of movement, viz. something short of a quadrant.

In this balance (manufactured by Messrs Milne, Edinburgh), the opposing arms are formed of grooved eccentric, or spiral curves. These are acted on by the cords or flat chains which suspend the counterpoise and receiving scale, and are thus caused to move round on their common axis, which is placed in the pole of the curves. In the annexed figure the dial is supposed to be transparent, and being fixed in front to the curved balance, moves along with it. The pointer is stationary, and is fastened to the shears or fork which suspends the balance, and indicates on the dial as it passes round. By the changes of leverage which occur in the course of revolution, the counterpoise rises from a small power to a greater; while the scale, in proportion as it becomes loaded, contracts throughout from a greater leverage to a less. This general [7:21:838]principle of combined changes of leverage affords a favourable means of employing an unchanging weight to balance others, both greater and less than itself; and at the same time admits of the parts necessary to produce powerful and sustained resistance being practically formed within convenient compass. The self-adjusting power thus generally obtained is regulated in the rate of its increase by a definite construction of the curves, which are formed to cause equable as well as progressive resistance, by which means the balance is moved through equal angles by equal weights. The graduated scale extends to three quadrants, more or less. The chief advantage of the balance is, that it combines this lengthened primary movement with equable graduation and unyielding mechanical construction. The different sizes of which it is made have each an appropriate form of curve. The least is fitted for the minute subdivision of small weights; the next indicates to four or sixteen ounces; and larger sizes range to twenty, fifty, 100 pounds and upward. In the adjoining figure, the spiral arms, which before are in the same vertical plane, and merge into one continuous curve, here occupy different planes; but being united by an intermediate rod, which passes through the upright of the stand, they have simultaneous movement The dial is fixed to the bearing-frame, and the pointer is carried round by the axis. The graduation, which in this form may be prolonged to any desirable extent, occupies a full circuit. In other respects, the description already given is alike explanatory of both balances. There are some minutiae which give refinement to their action; but as these are used only in particular machines, they are not represented, and need not be described in detail. A delicately constructed modification of the instrument is also arranged to shew, on inspection, and without the use of eke-weights, the specific gravities and weights per cubic inch of different fluids.

III. Self-adjusting Balances, which derive their resisting Power from the Elasticity of Metals.

Under this head may be mentioned Coulomb’s torsion-balance, formed of brass wire, the twisting and untwisting of which is applied to the measurement of small forces. It is described under Balance , p. 307. Hydbodynamics , p. 76, and Mechanics, p. 401. The dynanometer for the measurement of larger forces, and applicable also to the ascertainment of weight, is explained at the conclusion of the article Dynamics. That of Mr Veitch is represented and described in the article Agriculture, p. 355.

The spring is supposed to have been applied to the purposes of weighing about the year 1690 or 1700. About this period the dynanometers of Graham and Desaguliers were superseded, first by Monsieur Leroy’s of the Academy of Sciences, who applied to the purpose the compression of a spiral spring; and next by Monsieur Regnier’s, in which the distention of an elliptical one was adapted to the same end, both springs being then also in use as balances. The spring-balance is constructed in a variety of modified forms; but in all of them the weights under examination are caused either to bend a plain spring, or to compress or extend an elliptical, semi-elliptical, or spiral one; or both to compress and extend the spring when formed to admit of this. The amount to which any of these descriptions of spring yields to different stated weights, is shewn on a graduated scale properly situated for the purpose. The principle of extension is preferable to the other, inasmuch as it is put in operation by simpler and less complex means; and when the weights to be examined are large, the elliptical spring should be adopted; but when these are of a moderate amount, the spiral form is best. For many purposes the spring-balance is a convenient instrument, but not a lasting one. The spring, when fresh and unused, is equable in its successive yieldings, but is liable to variations, which cause it gradually to lose correspondence with the divisions. It is affected by temperature, and weakened by use; and if overstrained or corroded, its power and quality are materially changed. In some balances an extra spring is introduced to adjust these irregularities; but this should be omitted, as its operation increases the chance of error, without removing the cause; for though the pointer, when displaced, may he restored by it to zero, there are then two acting springs changed in position, quality, and strain, while the line of graduations remains as it was derived from the original elasticity of the leading spring.

In this balance, by Mr Marriott of London, the spring is formed into an arbitrary curve, and is placed in a shallow brass box behind the dial, which latter is here supposed to be transparent; one end of the spring is fixed to the upper side of the box, and the other end to the head of the rod which carries the hook. Near the same point of this rod is also attached the head of the rack-plate, which, as the hook becomes loaded, is lowered by the spring and turns the pinion, and with it the pointer, which indicates the weight.

When in use, the portion of the spring to the left is extended, and that to the right compressed; and during descent, the rack is kept in gear with the pinion by a screw-pin at its right side.

This balance, by Mr Salter of London, is represented in the annexed figure, the index-plate in front being supposed transparent. It is formed of a spiral spring, which is contained in the upper part of a cylindrical case, behind the index-plate. One extremity of the spring is fastened at the head of the instrument, and the other extremity is fixed to the rod which carries the pointer and the hook or scale. The spring extends in proportion as the hook is loaded; the pointer at the same time shewing the weight on the graduated front. Mr Salter subsequently made some alterations, which he patented; most of them however relate to minor details, and need not be described.

IV. Self-adjusting Balances, whose progressive Indications are regulated by the Properties of Fluids.

Under this head may be included several of the various instruments for ascertaining specific gravities, which are described under Hydrodynamics, p. 22-25. Although most of these require eke-weights to complete their range of indications, they are nevertheless of self-adjusting character; their principle of adjustment depending on the different depths to which fluids of varied density permit the graduated stem of the instruments to sink. Here also may be mentioned Mr Harris’s electrometer, described under Electricity, p. 658, a balance in which the resistance is obtained by the accumulating power which an elongated counterpoise acquires when caused to emerge from a fluid; the same principle being further illustrated under the article Gas-Light, p. 353—54, where its application to the progressive balancing of the gasometer is explained.
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This balance, by Mr Hawkins of London, consists of a cylindrical vessel open at top, on the face of which is a vertical glass tube also open above, and entering the cylinder near its base. Within this outer case is another hollow cylindrical drum, of rather smaller diameter, on the top of which is placed the receiving scale. The inner cylinder floats on the water which the outer one contains. In proportion as the scale is loaded, the water rises between the cylinders and in the glass tube, beside which is a graduated scale, the weight being indicated by the height which the fluid attains in the tube. The water carried off by evaporation should be replaced, as much being poured in from time to time as is required to preserve its surface at zero when the scale is empty. The continual effort of the inner cylinder to escape to the sides of the outer one is, however, a source of considerable friction; a slight displacement of the cylinder from the horizontal position, or the loading of the scale more on one side of its centre than another, contributing to augment the evil.

This balance, by Mr Bursill of Islington, is formed in a variety of different modifications; some of them depending on a stem or piston being caused to sink in a fluid, and others on its being caused to rise from one. It may however be generally described as consisting of two cylinders of different diameter united by a connecting tube. From the top of the smaller cylinder rises a glass tube, at the side of which is a moveable index-plate, on which the graduations are marked. Mercury is poured to some depth into the cylinders, the smaller of which, together with the lower part of the glass tube, is then filled up with oil or coloured water, which rests on the surface of the mercury. A piston or stem, guided by a parallel movement, rises and falls in the larger mercurial holder. A counterpoise on one of the parallel rods balances the weight of the scale; and when the latter is loaded, the displacement of mercury in the larger cylinder causes the water to rise in the tube by more perceptible gradations, and indicate the weight. The balance is likewise arranged in various combinations with the steelyard, and two contrivances (one of which may be added to the machine) are provided with the view of compensating the irregularities caused by change of temperature. The cylinders for the mercury are formed of pipe-clay; and a stop-cock is placed at the head of the tube, to be opened during use to admit air, and closed afterwards to exclude dust and diminish evaporation.

This machine, by Captain Ericson of London, is represented in the annexed figure. It consists of a shallow circular box of cast iron, formed of two portions, an upper and under one; the interior diameters of which are somewhat different. The lower half of the box contains a piston, the neck of which passes freely through an aperture in the bottom, and is screwed to the horizontal bar of the triangular suspending frame. A circular piece of India rubber cloth is laid over the face of the piston, and continued through between the joining rims of the iron box, the halves of which are then screwed together upon it. From the upper half of the box rises a hollow iron stem, sufficiently laid open to expose to view a glass tube, which it encloses in a bed of Paris plaster. Mercury is then introduced through the tube into the upper half of the box; and its escape being prevented by the cloth, and the distention of this resisted by the piston, the box and stem are set afloat, and are kept from capsizing by a small pin from the top of the triangular frame, which enters into the bore of the upright stem. The hook is connected with the box, and, on being loaded, draws it down, thereby causing the ring of cloth exterior to the circumfe rence of the piston to descend below the piston’s surface; —in effect thus contracting the space in the box, and forcing the mercury up the tube till it indicates the weight on the graduated scale. Two safety-screws in the lower bar are adjusted to prevent the box from descending so far as to eject the mercury from the tube; and, as in Mr Bur-sill’s, a stop-cock is placed at the mouth, to be opened during use to admit air, and shut after it to exclude dust and diminish evaporation. In this machine, the primary movement must necessarily be extremely confined (about the 50th or 100th part of an inch), otherwise the cloth, by being overstretched, would give way. Any deviation from uniform action, however arising, and however trifling, is thus apt to occasion a magnified irregularity in the indication. The admission of dust and evaporation of mercury; the effort made by this fluid at all points to draw into globular formations, and avoid the minuter recesses of the holder; the change of temperature, and progressive distention and wrinkling of the cloth, are among the more immediate and least controllable sources of variation.

V. Letter and Post-Packet Balances.

Under this general title are enumerated such of the preceding as are applied to the purpose of weighing letters; descriptions of the balances not yet noticed being subjoined.

1. The Common, the Roman, and Danish balances; 2. the Bent-lever, Brady’s, Dampier’s, and Lothian’s balances; 3. Salter’s and other Spring-balances; 4. Bursill’s balance, all as above described, are constructed also of sizes suited for letter-weights.

In this balance, which was devised for letter-weighing by Sir John Robison, secretary to the Royal Society, Edinburgh (and made by Mr Forbes there), the letter is placed on a circular ivory disc, which is supported by a steel wire, that passes freely through a small orifice in the ivory cap of a glass tube filled about a third of its depth with mercury. To the lower end of the wire is screwed a cylindrical ivory stem of larger diameter than the wire, and marked with circular graduations. This sinks in the fluid in proportion as the disc is loaded; and the vertical movement is preserved by guide-pins, which diverge from the lower extremity of the stem. A loose ring of ivory serves the purpose of a floating pointer, while it prevents the eye being deceived by the glance of the fluid, or the images occasioned by its reflecting powers. Shortly afterwards, letter-balances of similar description were made by Messrs Miller, Dundee; Lund, Fleet Street; Osler, Birmingham; and Bursill, Islington; the three latter being registered designs.

The remaining balances may properly be arranged by themselves, as a class formed to indicate periodical amounts of weight, but not their intermediate subdivisions.

In the balance by Mr Riddle of London, weights of an ounce each are rested on the successive steps of a pyramidal support. A ring attached to one arm of a balance lifts the lowest, and proceeds to the others in their order; the postage corresponding to the number of weights raised being indicated by a pointer. In Mr Gye’s balance three rings fixed at proper heights give support to as many weights, which are attached by a small chain to the arm of a balance, each weight being lifted in succession, and the postage known by the number of weights raised. Mr Riddle’s arrangement, however, provides for a more exact movement.

This balance, by Professor Willis, Cambridge (made by Messrs HotzapffeD, is formed of two beams joined at some distance apart, so as to form a rectangular frame, which the: figure of Poupard’s balance above described will serve to illustrate. Here also, as in that balance, the receiving scale [7:21:840]is placed above the beam, and is similarly guided in its descent by a parallel movement placed in the stand. The two beams are angularly notched in the manner shewn in the adjoining figure, which represents a side plan of the balance. Five cylindrical bars of adjusted weight rest in the arms of a rack, which supports them horizontally across each pair of notches, and at increasing elevations from the stand. These bars successively take their appropriate place on the double beam, and are each lifted by, and become as it were a part of it, so soon as the load placed on the scale exceeds the stated amount. Thus, when a letter exceeds half an ounce, the beam rises to, but does not lift, the first bar; when it exceeds an ounce, it lifts the first, and rises to, but does not raise, the second; and so on to the fifth. When it exceeds this, an eke-weight which accompanies the machine is placed in the square-shaped notches at the extremity of the beam, and five ounces or their corresponding postage are added to the subsequent indications of the balance, which anew traces the same progression. The weights or postages are marked on an upright which is fixed at one end of the stand, and are pointed out by a wire extended between the two beams of the balance.

In this balance, by Mr Freeman, Sutton-Common, there are various fixed angular fulcra and a constant weight. The beam, which is loaded at one end, rests on the angular fulcrum nearest the weight. The fulcrum is supported at each end by a frame which rises from the top of a pillar. The upper surfaces of the two supporting frames are obtusely rounded, and notched at intervals suited to receive each fulcrum in its turn. When a letter exceeds a half ounce, the beam turns on, but does not quit, the first fulcrum. When it exceeds an ounce, it turns on, but does not quit, the second, and so on; the resting extremities of each successive fulcrum taking their appropriate place in a new pair of notches, and a pointer below shewing the result. This balance in general conception is a counterpart, or very nearly so, to Mr Willis’s, but is one which perhaps scarcely admits of being practically formed for very effective operation.

The theory of balances depending on the properties of the lever is given at large under the articles Mechanics, pp. 358, 370, and Balance ; that of the spring is illustrated under Mechanics, p. 402; and the theory of those depending on the properties of fluids is fully explained under Hydrodynamics, pp. 13, 14, and 19-25. (d. ι.)