Encyclopaedia Britannica, or, a Dictionary of arts, sciences, and miscellaneous literature : enlarged and improved. Illustrated with nearly six hundred engravings > Volume 5, BUR-CHI

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Burning. as ;n tlie otlier cafe ; while the effects of it muft be
v_ much greater, on account of the greater thinnefs of
the glafs.
M. Trudaine, a French gentleman, conftru&ed a
burning lens on a new principle. It was compofed of
two circular fegments of glafs fpheres, each four feet in
diameter, applied with their concave Tides towards each
other. The cavity was filled with fpirit of wine, of
which it contained 40 pints. It was prefented by the
maker to the royal academy of fciences, but was, not
long after, broken by accident. The expence of con-
ftrudling it amounted to about toool. fterling. After
all, it does not appear that the effefts of this lens were
very great. Mr Magellan informs us, that it could on¬
ly coagulate the particles of platina in 20 minutes,
while Mr Parker’s lens entirely melted them in lefs
than two.
A large burning lens, indeed, for the purpofe of fu-
ling and vitrifying fuch fubftances as refill the fires of
•ordinary furnaces, and efpecially for the application of
heat in vacuo, and in other circumllances in which heat
cannot be applied by any other means, has long been a
defideratum among perfons concerned in philo’fophical
experiments : And it appears now to be in a great de*
gree accomplilhed by Mr Parker. His lens is three
feet in diameter, made of flint-glafs, and which, when
fixed in its frame, expofes a furface two feet eight
inches and a half in the clear.
In the Elevation reprefented on Plate CXXXII, A is
the lens of the diameter mentioned : thicknefs in the
centre, three inches and one-fourth : weight, 212
pounds : length of the focus, fix. feet eight inches ;
diameter of ditto, one inch. B, a fecond lens, whofe
diameter in the frame is 16 inches, and {hows in the
clear r 3 inches : thicknefs in the centre, one inch five-
eighths: weight 21 pounds: length of focus 29 inches:
diameter of ditto, three-eighths of an inch. When the
two above lenfes are compounded together, the length
of the focus is five feet three inches: diameter of ditto,
half an inch. C, a truncated cone, compofed of 21
ribs of wood: at the larger end is fixed the great lens A;
at the fmaller extremity the leffer lens B : near the
fmaller end is alfo fixed a rack D, paffing through the
pillar L, moveable by a pinion turning in the faid
pillar, by means of the handle E, and thus giving a
vertical motion to the machine. F, a bar of wood,
fixed between the two lower ribs of the cone at G ;
having, within a chafed mortice in which it moves, an
apparatus H, w ith the iron plate, I, fixed thereto; and
this part turning on a ball and focket, K, a method is
thereby obtained of placing the matter under experi¬
ment, fo as to’ be afted upon by the focal rays-in the
moll dire£l and powerful manner. LL, a flrong ma¬
hogany frame, moving on callors, MM. Immediately
under the table N are three fritlion wheels, by which
the machine moves horizontally. O, a llrong iron
bow, in which the lens and the done hang.
SeFlion.—a, The greai lens marked A in the eleva¬
tion. b, The frame which contains the lens, c, The
fmall lens marked B. d, The frame which contains
the fmall lens, e, The truncated cone, marked C.
f, The bar on which the apparatus marked F moves.
g, The iron plate marked I. h, The cone of rays
formed by the refradlion of the great lens a, and falling
on the lens c. i, The cone of rays formed by the
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refraction of the lens c. Ft ont-view.—k, The great Burnifif.
lens. 1, The frame containing it. m, The llrong iron
bow in which it hangs.
From a great number of experiments made with this
lens, in the prefence of many fcientific perfons, the
following are feleCled as fpecimens of its powers.
Subltances fufed, with their weight and
time of fufion.
ao w
16
- c
Gold, pure,
Silver, do.
Copper, do.
Platina, do.
Nickel],
Bar iron, a cube,
Call iron, a cube,
Steel, a cube,
Scoria of wrought iron,
Terra ponderofa, or barytes,
A topaz, or chryfolite,
An oriental emerald,
Cryltal pebble,
White agate,
Flint, oriental,
Rough cornelian,
Jafper,
Onyx,
Garnet,
White rhomboidal fpar,
Zeolites,
Rotten llone,
Common flate,
A.lbeltos, - -
Common lime-llone,
Pumice-Rone,
Lava, - -
Volcanic clay,
Cornilh moor-flone,
20
20
33
10
16
10
10
10
12
xo
3
2
7
10
ro
10
10
10
10
10
10
JO
10
10
10
10
10
10
10
3
4
20
3
3
12
3
12
2
7
45
25
6
30
3°
75
25
20
17
60
23
80
55
24
7
60
60
Burning Mountains. See /Etna, Hecla, Vesu¬
vius, and Volcano, with the plates accompanying
them.
BURNING Springs. Of thefe there are many in dif¬
ferent parts of the world 5 particularly one in Dau-
phiny near Grenoble ; another near Hermanfladt irt
Tranfylvania 5 a third at Chermay, a village near
Switzerland 5 a fourth in the canton of Friburg ; and
a fifth not far from the city of Cracow in Poland.
There alfo is, or was, a famous fpring of the fame
kind at Wigan in Lancalhire, which, upon the ap¬
proach of a lighted candle, would take fire and burn
like fpirit of wine for a whole day. But the moR re¬
markable one of this kind, or at leaR that of which we
have the moil particular defeription, wras difeovered in
1711 at Brofely in Shroplhire. The following account
of this remarkable fpring was given by the reverend Mr
Mafon, Woodwardian profelfor at Cambridge, dated
February x8. 1746. “ The w^ell for four or five feet
deep is fix or feven feet wide 5 within that is another
lefs hole of like depth dug in the clay, in the bottom
whereof is placed a cylindric earthen veffel, of about
four or five inches diameter at the mouth, having the
bottom taken off, and the fides well fixed in the clay
camme4