PYROMETER. 



was before, and then ict the index to the 360th degree 

 again. I-ight the lamp, and put it under the iron bar, and 

 let it remain jull as many feconds as it did under the brafs 

 one ; and liien blow it out, and you will fee how many 

 degrees the index has moved in the circle : and by that 

 means you will know in what proportion the cxpanfion of 

 iron is to the expanfion of brafs ; which will be found to 

 be as 3 10 is to 360, or as 7 to 12. By this method, the 

 relative expanfion oi different metals may be found. 



The bars ought to be exactly of equal fize, and to have 

 them fo, they Ihould be drawn, like wire, through a hole. 



When the lamp is blown out, you will fee the index turn 

 backward ; which fhews that the metal contrails as it 

 cools. 



The iiilide of this pyrometer is conltrufted as follows. 

 In Jig. 8. A a is the Ihort bar, which moves between 

 rollers ; and, on the fide a, it has fifteen teeth in an inch, 

 which take into the leaves of a pinion B (twelve in number), 

 on whofe axis is the wheel C of one hundred teeth, which 

 take into the ten leaves of the pinion D, on whofe axis is 

 the wheel E of one hundred teeth, which take into the ten 

 leaves of the pinion F, on the top of whofe axis is the index 

 above-mentioned. 



Now, as the wheels C and E have one hundred teeth 

 each, and the pinions D and F have ten leaves each, it is 

 plain, that if the wheel C turns once round, the pinion F, 

 and the index on its axis, will turn one hundred times round. 

 But, as the firit pinion B has only twelve leaves, and the 

 bar A a that turns it has fifteen teeth in an inch, which is 

 twelve and a fourth part more ; one inch motion of the bar 

 will caufe the laft pinion F to turn one hundred times round, 

 and a fourth part of one hundred over and above, which is 

 twenty-five. So that if A a be pufhed one inch, F will 

 be turned one hundred and twenty-five times round. 



A filk thread b is tied to the axis of the pinion D, and 

 wound feveral times round it ; and the other end of the 

 thread is tied to a piece of llender watch fpring G, which 

 is fixed in the ftud H. So that as the bar f expands, and 

 puflies the bar A a forward, the thread winds round the 

 axle, and draws out the fpring ; and as the bar contracts, 

 the i'pring pulls back the thre id, and turns the work the 

 contrary way, which puflies back the Ibort bar A a againit 

 the long bar_/". This fpring always keeps the teeth of the 

 wheels in contaft with the leaves of the pinions, and fo 

 prevents any fhake in the teeth. 



In ^g. 7. the eight divifions of the inner circle are fo 

 many thoulandth parts of an inch in the expanfion or con- 

 traction of the bars ; which is juit: one thoufandth part of 

 an inch for each divifion moved over by the index. Fer- 

 gufou's Leftures on Mechanics, Supplem. p. 7, &c. 410. 



Another pyrometer was invented by M. De Luc, in 

 confequence of a hint fuggeited to him by Mr. Ramfden. 

 The bafis of this inftrument is a rectangular piece of deal 

 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 fquare, 

 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 Hand ; the board being in a ver- 

 tical fituation in the' diretlion 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 w'ith a very confiderable degree of 

 lightnefs ; the friftion of which may be increafed by the 



prcffure of four fcrcws. The inner fiiding frame, which is 

 likewife of deal, keeps the tube of the microfcope in a 

 horizontal pofition, and in great part without the frame, 

 infomuch th.\t the end which carries the lens is but little 

 within the fpace between the frame and the board. This 

 microlcope is conitrudted in fuch a manner as that the 

 objeft obferved may be an inch diitant from the lens ; and 

 it has a wire which is fituated in the focus of the glaffes, in 

 which the objeifts appear reverfed. At the top of the 

 apparatus there is a piece of deal, an inch and a half thick, 

 and two inches broad, laid in a horizontal direftion from 

 the board to the top of the frame. To this piece the rods 

 of different fubffances, whofe expanfion by heat is to be 

 meafnred, are fufpended : one end of it flides into a focket, 

 which is cut in the thickncfs of the board, and the other 

 end, whicii reifs upon the frame, meets there with a fcrew, 

 which makes the piece move backwards and forwards, to 

 bring the objeft s to the focus of the microfcope. There is 

 a cork very ftrongly driven through a hole bored vertically 

 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 counteracted bv 

 any preiiure. In order to heat the rods, a cylindrical bottle 

 of thin glafs, about twenty-one inches high, and four inches 

 in diameter, is placed in the infide of the machine, upon a 

 iland independent of the reft of the apparatus. In this 

 bottle the rods are fufpended at a little lefs than an inch 

 diilance from one of the fides, in order to have them near 

 the microfcope. Into this bottle is poured water of dif- 

 ferent degrees of heat, which muft be ftirred about by 

 moving upwards and downwards, at one of the fides of the 

 bottle, a little piece of wood fattened horizontally at the 

 end of a flick : 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 water rifes to the cork, 

 which thus determines the length of the heated part ; the 

 bottle is covered to prevent the water from cooling too 

 rapidly at the furfacc ; and a thin cafe of brafs prevents 

 the vapour from fixing upon the piece of deal to which the 

 rods are fixed. This pyrometer is reprefented in _/fo-. 9, in 

 which a a is the ftand to which it is fufpended ; b the hook 

 from which it hangs ; c c c the deal-board, which is the bafis 

 of the whole apparatus ; d, d, d, d, four arms, to which is 

 fixed the frame eeee: the other frame which carries the 

 microfcope is s s s s : g, g, are two crofs-pieces, through 

 which paffes the tube of the microfcope, and which fupport 

 it near both ends ; /j A is the microfcope ; /' its micrometer ; 

 k the cork, through which paffes the glafs rod //, and by 

 which it is kept fufpended ; m a rod of metal, or any other 

 fubflance lefs dilatable than glafs ; n the point of union, 

 obtained by means of two connefted rings, in which both 

 rods are faflened by fcrews : above thefe is another pair of 

 rings, in one of which the metal rod is free, and which rod 

 it fupports ; op the piece to which the glafs rod is fuf- 

 pended ; q a fquare piece fixed to the frame by four fcrews, 

 behind which is a box, in which, as weU as in a groove cut 

 in the bafis in p, the piece op flides ; r a fcrew, which paffes 

 through the fquare piece q, whofe ufe is to move backwards 

 or forwards the piece y, in order to bring the furface of the 

 metal rod to the focus of the microfcope : j-, s, s, s, four 

 fcrews, with round metal plates behind their heads, which 

 ferve to prefs the frame of the microfcope againfl the 

 frame eeee ; the longitudinal openings, through which the 

 fcrews pafs, permit the free motion of the firfl frame, when 

 one flrikes gently with a hammer to the bottom or the top 

 of one of the fides. If the microfcope is wanted higher 

 or lower than the grooves permit, the fcrews may be re- 

 moved 



