i o 0 
M E C I I A N I C S. 
the plate, as the face of the plate points out the divifions 
on the index. When the inftrument is ufed, hold the 
knob of the feeler R with one hand, and, moving it up 
and down, with the other move forward the fcrew T, till 
its point comes in contaft with the feeler; then with the 
plate and index, V and X, (how the turns and parts. Fig. 44 
reprefents the inltrument immerfed in a cittern of water, 
ready for ufe. A B is the cittern ; C the cover, which, 
when the inftrument is raifed upon blocks, goes on be¬ 
tween the bar E F and the bafis B C, fig. 43. D is a 
handle to take off the cover when hot. E, a mercurial 
thermometer, whofe ball is in the water. F, a cock to 
let out the water. G H a hollow piece of tin, which fup- 
ports feven (pint-lamps, which are raifed or depreffed by 
the fcrews I and K, to give the water a proper degree of 
heat, fliown by the thermometer, E. 
Pyrometers conftrufted to aft by levers are better than 
thofe by wheel and pinion. The expanlion of a bar, 
w hofe body is fimply placed under two microfcopes fixed 
to a bar of calt-iron, conftantly kept to the freezing tem¬ 
perature by enveloping it with melting ice, is found to be 
more accurately obferved than by any other inftrument. 
Pyrometers on thefe conltruftions ihow the expanfion from 
the 50000th to the 100000th of an inch. Adams’s LcElures , 
vol. i. p. 296 & feq. 
The following Table (hows how much a foot in length 
of each metal grows longer by an increafe of heat corref- 
ponding to 180 0 of Fahrenheit’s thermometer, or to the 
difference between freezing and boiling water, expreffed in 
fuch parts of which the unit is equal to the 10,000th part 
of an inch. 
White glafs barometer tube - - 100 
Martial regulus of antimony - 130 
Blittered fteel .... 138 
Hard fteel - 147 
Iron - - - - - 151 
6. Bifmuth - 167 
7. Copper hammered ... 204 
8. Copper eight parts with tin one - - 218 
Caft-brafo ... . 225 
Brafs lixteen parts with tin one - - 229 
Brafs wire, and fpeculum-metal - 232 
Spelter folder, viz. brafs two parts, zinc one 247 
Fine pewter .... *74 
Grain-tin ..... 298 
Soft folder, viz. lead two, tin one - 302 
Zinc eight parts, with tin one, a little hammered 323 
Lead - 344 
Zinc or fpelter - - - - 3 53 
Zinc hammered half an inch per foot - 373 
M.de Luc, in confequence of a hint fuggefted to him by 
the late Mr. Ramfden, invented a pyrometer, the bafis of 
-which is a reftangular piece of deai board, two feet and a 
half long, fifteen inches broad, and one inch and a half 
thick ; and to this all the other parts are fixed. This is 
mounted in the manner of a table, with four deal legs, 
each a foot long and an inch and a half l'quare, well fitted 
near its four angles, and kept together at the other ends by 
four firm crofs-pieces. This fmall table is fufpended by a 
hook to a ftand ; the board being in a vertical fituation in 
the direftion of its grain, and bearing its legs forward in 
fuch a manner as that the crofs-pieces which join them 
may form a frame, placed vertically facing the obferver. 
This frame fuftains a microfcope, Which is firmly fixed in 
another frame that moves in the former by means of 
grooves, but with a very confiderabie degree of tightnefs ; 
the friftion of which may be increafed by the preflure of 
four fcrews. The inner iliding-frame, which is likew-ife 
of deal, keeps the tube of the microfcope in a horizontal 
pofition, and in great part without the frame, infomuch 
that the end w hich carries the lens is but little within the 
fpace between the frame and the board. This microfcope 
is conflrufted in fuch a manner as that the objeft obferved 
may be an inch diftant from the lens; and it has a wire 
which is fituated in the focus of the glafTes, in which the 
®bjefts appeared revelled. At the top of the apparatus 
1. 
2. 
3 - 
4 - 
5 - 
9 - 
■JO. 
•11. 
12. 
33 - 
.14. 
35- 
16. 
3 7 - 
.18. 
19. 
there is a piece of deal, an inch and a half thick and twa 
inches broad, laid in a horizontal direction from the board 
to the top of the frame, To this piece the rods of the 
different fubftances, whofe expanfion by heat is to be mea- 
fured, are fufpended : one end of it Aides into a focket, 
which is cut in the thicknefs of the board ; and the other 
end, which refts upon the frame, meets there with a ferew 
which makes the piece move backward and forward, to 
bring the objects to the focus of the microfcope. There 
is a cork very ftrongly driven through a hole bored verti¬ 
cally through this piece; and in another vertical hole 
made through the cork, the rods are fixed at the top ; fo 
that they hang only, and their dilatation is not counter- 
afted by any preflure. In order to heat the rods, a cylin¬ 
drical bottle of thin glafs, about twenty-one inches high and 
four inches in diameter,is placed in the in fide of the machine, 
upon a (land independent of the reft of the apparatus. 
In this bottle the rods are fufpended at a little lei's than att 
inch diftance from one of the infides, in order to have them, 
near the microfcope. Into it is poured water of different 
degrees of heat, which mull be ftirred about, by moving 
upwards and downwards, at one of the tides of the bottle, 
a little piece of wood, fattened horizontally at the end of 
a (tick. : in this water is hung a thermometer, the ball of 
which reaches to the middle of the height of the rods. 
During thefe operations the rvater rifes to the cork, which 
thus determines the length or the heated part. The bot¬ 
tle is covered, to prevent the water from cooling too ra¬ 
pidly at the furface; and a thin cafe of brafs prevents the. 
vapour from fixing upon the piece of deal to which the 
rods are fixed. 
The late Mr. Fergufon alfo invented two pyrometers, 
deferiptions and figures of which are given in his Lec¬ 
tures. 
Mr. Wedgwood, the ingenious manufafturer of the 
fineft earthen-ware from balaltic mattes, or terra cotta, 
has contrived a curious pyrometer, which marks, with 
much precifion, the different degrees of ignition, from a 
dull red heat vifible in the dark to the heat of an air-fur¬ 
nace. Its indications are obtained from the property 
which all days poffefs, of Shrinking or being diminifheci 
in bulk by expolure to heat, (contrary to metals, which 
in the fame fituation expand;) (o that confequently any 
piece of well-dried clay fitted into a tapering gauge, (for' 
inflance, a leftor opened a little,) will, after expolure to a 
iufficient degree of heat, pals further into the gauge; and 
that, the greater the degree of heat it has experienced, 
the greater will be its diminution of volume, and the 
deeper will it go into the gauge. Different divifions 
marked on the fide of the gauge, will of courfe give the 
comparative degrees of heat to which pieces of the kune 
clay, fize, and form, may have been expofed. From Mr. 
Wedgwood’s own experiments it appears, that the clay 
lie made ufe of in the conltruftion of his firft pieces con- 
fitted of tw>o parts of filiceous earth to three argil. We 
mention this, becaule it is pretty generally believed, that 
the pyrometer-pieces which have of late been prepared 
do not give the fame refults with thofe which were firft 
made by Mr. Wedgwood ; a circumftance which makes 
it extremely defirable that the quantities and kind of 
earth neceflary to form fuch rolls as will always give cor- 
reft refults fliould be accurately afeertained. The inftru¬ 
ment itfelf is extremely liniple. It confilts of two rulers 
fixed upon a fmooth fiat plate, a little farther afunder at 
the one end than at the other, leaving an open longitu¬ 
dinal fpace between them. Small pieces of alum and clay 
mixed together are made of fuch a fize as juit to enter at 
the wide end ; they are then heated in the fire along with 
the body whole heat we wifh to determine. The fire, ac¬ 
cording to the degree of heat it contains, diininilhes or 
contradts the earthy body, fo that, when applied to the 
wide end of the gauge, it will Aide on towards the nar¬ 
row end, lefs or more, according to the degree of heat to 
which it has been expofed. 
That this inftrument may be perfeftly underftood, we 
have given a reprefentation of it. ABC D, fig. 45. is a 
imootli 
