The republic. [volume] (Washington [D.C.]) 1849-1853, 1852-05-15
From the London 1then9um.
RnVuI InstitnfiOn

rock, and the Skerrevore lighthouses and the phi
| losophy Of their brilliant light render them emf
| nent objects of that scientific interest which be-
longs TO all similar structures. The Eddystone
lighthouse, having been built of wood in 1893, was
carried away five years after its erection. II was
shcrly after reconstructed OF the same material,
the lower part being filled with stone concrete;
it then lasted for forty years, when it was con
| sumed by fire. In 1859 Smeaton completed the
present lighthouse which is IS feet high, and the
base rs feet in diameter, (beng barely less than
the surface of the rock which it stands.) 1t ID
built of stone; the stones dovetailed together,
and '.joggled" as it is termed, to prevent the
courses OF stones from sliding on each other II is
suated in the midst of the sea, nine or ten miles
distant from Plymouth. The Bedrock lighthouse
stands rock Of the same name the east coast
of Scotland. 1t ID surrounded by the and is 100
feet high, and AZ feet in diameter at the base IL
was built by Robert Stey and finished in 1870
lts construction is similar to that OF the Eddystone
The Skerrevore lighthouse was built by Alan Ste
venson, of the architect Of the Bedrock light
house. The mass Of stone in this structure Is
| more than double that used in the Bedrock, and
| five times that contained in the Eddystone. The
tower is l3S feet high and the diameter at the base
Is "A feet. 1t stands on gneiss rock, the area OF
which is just large enough for the foundation. In
constructing this lighthouse, the architect appears
to have chiefly relied on the weight, rather than
on the extension of the materials, for efficient re-
sistance to the impact OF the waves. The stones
were not dovetaed or Joggled but tree nails were
| used merely to keep the work together during its
| erection. Several lighthouses have Of late years
| Mr.. Alexander Gordon, and made by Messrs
Cottam s. Hallen, has been erected at Bermuda;
! it is $50 feet high Messrs. Walker & Burgess
have recently constructed efficient lighthouses
iron piles which fixed in the sand by means of
| a screw, invented by Mitchell The Maplin and
Chapman lights, at the mouth of the Thames, and
those at Fleetwood and Belfast, on this prin-
clple. Prof Cowper invited attention to the mode
in which these structures are rendered compact by
means OF cast-iron braces. The Sources of Light
and mode OF diilusing it were next adyerted 10.
| Common fires, first OF wood, and then OF Goals,
were originally used to furnish light coal fire
was employed for this purpose in the Isle OF May
| for one hundred and eighty years Gas late as the
! year 1850 Tallow candies succeeded-candles
| fastened on wooden rods Gas they sometimes
! seen arranged before booths in iairs) were burnt
| in the Eddystone lighthouse for forty years alter
it was completed by Smcaton. Then came lamps
| with twisted-cotton wicks, and then common
| gand lamps; all theee, however. are now super
seeded by (A) argand lamps and reiiectors, OBJ
one argand lamp, with lenses and rehectors. and
| co one argand lamp, with lenses and rejecting
prisDs. Mr.. Cowper illustrated the laws OF refec-
ton by several models, diagrams and familiar ex-
amples. As instances of refraction, he alluded to
the line of light produced on ripping water by the
rays of the sun or moon; each wave may Tin com
mon with every curved suriace) be considered as
| polygon having an infinite number OF sides; there
must therefore, be some side In such position as
will reflect the light. The same effect was pro
duccd by row of glass rods placed side by side.
Reference also made TO looking glass casting
the sunshine on the wall-to rei,ectors placed at
window exhibit objects in the strect-to the glow
in the sky produced by burning house; this ap
pearance being half way between the spectator and
the conflagration, occasions continual mistakes
~ the locality of the fire.
A Argand Lamps and Refectms Having ex-
empllhed the principle upon which light rs reflect
| ed, Mr.. Cowper demonstrated, by means of
| ries of small mirrors which movable on
fixed axis, that if light were placed in the locus
of paraboloid the rays would be reflected paral-
lei This is done in those lighthouses where
| Electors employed. The difficulty of shaping
parabolods was referred 10, and it was mentioned
that they were raised from fat sheet of metal by
the hammer. The arrangement of the lamp and
| reflector was described; and the halo like diffusion
Of the light consequent on the impossibility OF con-
centrating the luminous point in the focus Of the
parabola, was noticed. la tonne rion with this
part of his subject, Mr.. Cowper dwet on the dis
tinction between fired and reu0iuing light. The
former, being intended Ia be visible all round
the horizon, requires more lamps than the lat
ter; when three rows OF twelve lamps, each row
being in contact arc arranged in circular
form. the three lamps which in vertical
line Immediately opposite to the spectator afford
strong light, while the three either side less
distinctly seen, the parallel rays described not
reaching his eye With respect to the rerolging
iights-supposing twenty eight lamps arranged on
the four sides OF paralelopipedon, them as the
figure revolvcs, each eide will present seven lamps
| in succession. These, by shining at once, will
produce much stronger light than the fixed light.
The duration OF this effect WILL however, be short;
| because, each side is turned away from the
spectator the light will decrease rapidly; this will
| be succeeded by darkness, and THE darkness will
IN its turn be dispersed by rapidly INCREASING
light. Mr.. Cowper proceeded TO state that go salts
factory had been tho result of metal retectors in
lighthouses, that there seemed small scope for i'm.
provcment, until Fresnel devised the application Of
lcnsee, and also rejecting prisms in combination
with lenses, TO a single large lamp. To make this
invention clearly understood, Mr.. Cowpcr
planned the general laws of the reflection of light,
and illustrated his explanations by various dia
grams and models.
noa .,1A II. fIUU
| Mirrors Having shown that light on passing
| through n triangular prism Of glass Is refracted
towards its base, MIR Cowper applied this principle
TO the construction Of lcus which he derived from
two long thin prsms placed base to base. He de-
mopstrated that dyergng rays of light, admitted
on one side such solid would issue parallel on
eohcr side. There are great practical difficulties
| in fabricating n large glass lens. Condorcet and
Brewster suggested, and FTcsncl Effected the con-
structon OF lens Of separate prisIns, all unneces.
sary glass being removed. Diagrams OF such
lenses were shown; and il stated that they
were uscd with single largo lamp placed in the
focus Of the lens In this position. however, az was
! shown, all the rays which passed above and be.
meath the lens might escapc. Toinierccp. the rays,
FroBnel placed svered glass mirrors at the proper
angles. Fresnel alec made polygon Of straight
lentcular prisms, producing long line of strong
| light; but tho greatest improvement efieoc1 by
this great philosopher was, the substitution OF re-
| ftcing pris,ns for mirrors, thus introducing:
&. The Principle OF Lighting by one Argdnd Lamp,
| Lonsts, and Rejecting Ptisms. -Nlr. Cowper here
| ueInonsubIcd, Dy an apparatus COntriVcu Tor the
purpose, that when light IA incident the second
surface OF prism, II may fill obliquely that the
surface cannot reiract II, and that therefore tbs
incident light is totally r.fecied from the second
surface. Thus, II a ray enters the glass prism so
rs TO make the angle OF incidence greater than 4lo
| 49,, n IL totally reflected. Mr. Co~pcr showed
| how this principle is applied in lighthouses. lie
stated that the first light OF THE kind, large
scale, was put up 10y Alan Stcvcnson at tho Skerrc
vorcj and that in 1913 Fresnel tried its ilhrnI
mating powers against those OF mirrors, aud found
the superiority to be in the proportion Of I0 to n
'On this subject" said MIR. COwper, IN conclusion,
zone ID struck with the intensity and exclusivencss
of thought devoted TO each part of the whole matter
| The Admiralty intensely desire lighthouse
particular spot. The engineer Is intensely occupied
| in surveying, loyclling, and building and, with "A
perseverance almost superhuman, ne continues his
! work during two or three years on the edge Of
| rock just showing itsclf above the waves. lie
| makes temporary barrack wooden piles on
| some adjacent point This is oil swept away in
ono night. lie builds il again, and il obliged 20
live In 1t for fourteen days together, the weather
preventing nil access to IT, Presently, however,
tower one hundred aud hrty.cight feet high
stands securely fixed on the exact spot assigned TO