'pyromp:ter. 



It is to be obferved of tin, that it will eafily melt when 

 heated by two flames placed together. Lead commonly 

 melts with three flames placed together, efpecially if they 

 burn long. 



From thefe experiments it appears, at firlt view, that 

 iron is the leaft rareiicd of any of thefe metals, whether it 

 be heated by one or more flames ; and therefore is moft 

 proper for making machines, or initruments, which we 

 would have free from any alterations by heat or cold, as 

 the rods of pendulums for clocks, &c. So likewife the 

 meafures of yards or feet fliould be made of iron, that 

 their length may be as nearly as poffiblc the fame, fummer 

 and winter. 



The expanfion of lead and tin, by only one flame, is 

 nearly the fame ; that is, almofl: double of the expanfion of 

 iron. It is likewife obfervable, that the flames, placed to- 

 gether, caufe a greater rarefaftion than when they have a 

 fenfibb interval between them ; iron, in the former cafe, 

 being expanded 117 degrees, and only 109 in the latter; 

 the reafon of which difterence is obvious. By comparing 

 the expanfion of the fame metal, produced by one, two, 

 three, or more flames, it appears, that two flames do not 

 caufe double the expanfion of one ; nor three flames three 

 times that expanfion, but always lefs ; and thefe expanfions 

 difler fo much the more from the ratio of the number of 

 flames, as there are more flames afling at the fame time. 



It is alfo obfervable, that metals are not expanded equally 

 at the time of their melting, but fome more, fome lefs. Thus, 

 tin began to run, when rarefied 219 degrees ; whereas brafs 

 was expanded 377 degrees, and yet was far from melting. 



As to the confl;ru<?tion of M. Mufchenbroeck's pyro- 

 meter, together with Defaguliers's alterations and improve- 

 ments, the curious may confult Defagul. Experim. Philof. 

 vol. i. p. 421, &c. See alfo Mufchenbroeck's tranflation 

 of the experiments of the Academy del Cimento, printed at 

 Leyden in 1731. And for a pyrometer of a new confl:ruc- 

 tion, by which the dilatations of metals in boiling fluids may 

 be examined and compared with Fahrenheit's thermometer, 

 fee Mufchenb. Introd. ad Philofophiam Nat. 4to. 1762, 

 vol. ii. p. 61S. 



But it has been obferved, that M. Mufchenbroeck's py- 

 rometer was liable to fome objeftions ; and a contrivance 

 was made, with a view of removing thefe, by Mr. Ellicott, 

 who has given a defcription of his improved pyrometer in 

 the Philofophical Tranfaftions, N° 443. This may alfo be 

 feen in Dr. Martyn's Abridgment, vol. viii. p. 464. 



This infl:rument is formed with a flat piece of brafs A A, 

 (P/rt/f XXIII. Mifcellany, Jig. /^.) which is fcrewed down 

 to a thick piece of mahogany. Upon this plate are fcrewed 

 three pieces of brafs, two of which, B, B, ferve to fupport 

 the flat iron bar C, called the ftandard bar. The upper 

 part of the third piece of brafs is a circle D, about three 

 inches in diameter, divided into three hundred and fixty 

 equal parts, or degrees : within this circle is a moveable 

 plate d, divided likewife into three hundred and fixty parts, 

 and a fmall fteel index. The bar of metal E, upon which 

 the experiment is to be made, is laid on the ftandard bar. 

 F is a lever two inches and a half long, faftened to an axis, 

 which turns in two pieces of brafs, fcrewed to one of the 

 fupports B. To the end of this lever is failened a chain, 

 or filk-line, which, after beiag wound round a fmall cy- 

 linder, to which the index in the brafs circle, D, is faitencd, 

 pafles over a pulley, with a weight hung to the end of it. 

 Upon the axis, to wliich the lever is fixed, is a pulley, a 

 quarter of an inch in diameter, to which a piece of watch- 

 chain is faftened, the other end of which is hooked to a 

 ■ ftrong fpring G, which bears againft one end of the metal E, 



H is a lever, exaftly of tlie fame form and dimenfions with 

 the other ; but the chain faftened to the pulley on its axis 

 is hooked to the ftandard bar. The line faftened to the end 

 of this lever, after being wound round a cylinder, to which 

 the moveable plate is fixed, pafles over a fmall pulley, and 

 has a weight hung to the end of it ; or rather the fame line, 

 pafling under a pulley to which the weight is hung, has its 

 other end faftened to the lf-»er F ; fo that one weight ferves 

 for both levers. From this defcription it is plain, that 

 whenever tlie bar/E is lengthened, it gives liberty to the 

 weight to draw the lever I" upwards by its adtion to the 

 fpring G ; and the index will, at the fame time, by means 

 of the filk line, be carried forward in the circle ; and as the 

 bar fliortcns, it will return back again : the fame motion 

 will be communicated to the ftandard bar. When the bar 

 is lengthened the 20th part of an inch, the index will be 

 carried once round the brafs circle, whicli is divided into 

 three hundred and fixty degrees ; and, therefore, if the 

 metal lengthens the 720odth part of an inch, the index will 

 move one degree. In order to make an experiment with 

 this inftrument, lay a bar of any kind of metal, as E, on 

 the ftandard bar ; then heat this bar to any degree of heat 

 with a lamp, and mark the degree of its expanfion, as indi- 

 cated by the moveable plate ; obferve alfo the degree of 

 expanfion of the metal E, by the heat communicated to it 

 from the ftandard bar, as marked on the brafs circle by the 

 index : let the inftrument ftand, till the whole is thoroughly 

 cold ; then removing the bar E, lay any others fucceffively 

 in its place, and proceed exaftly as before; and_thus the 

 degrees of expanfion of different metals, by the fame degree 

 of heat, may be eftimated. 



By the help of this inftrument Mr. Ellicott found, upon 

 a medium, that the expanfion of bars of different metals, as 

 nearly of the fame dimenfions as poffible, by the fame degree 

 of heat, were as follow : 



The great difference between the expanfions of iron and 

 brafs, has been applied with good fuccefs to remedy the ir- 

 regularities in pendulums arifmg from heat. Phil. Tranf. 

 v»l. xlvii. p. 485. See Pesdulum. 



Mr. Graiiam ufed to meafure the minute alterations, in 

 length, of metal bars, by advancing the point of a micro- 

 meter-fcrew, till it fenfibly ftopped againft the end of the 

 bar to be meafured. This fcrew, being Imall and very 

 lightly hung, was capable of agreement within the three or 

 four thoufandth part of an inch. On this general principle 

 Mr. Smeaton contrived his pyrometer, in which the mea- 

 fures are determined by the contatl; of a piece of metal with 

 the point of a micrometer-fcrew. A BCD {Jig- 5.) re- 

 prefents the main bar or bafis of this inftrument ; E F is the 

 bar to be meafured, lying in two notches, one fixed to the 

 upright ftandard A B, the other to the principal lever HI; 

 the end E of the bar E F bears againft the point of G, a 

 fcrew which is of ufe in examining the micrometer-fcrew ; 

 the other end of the bar F bears againft a fmall fpherically 

 protuberant bit of hard metal, fixed at the fame height as 

 G, in the pi-incipal lever HI; K is an arbor fixed in the 

 bafis, which receives at each end the points of the fcrews, 

 H, L, upon which the lever H I turns and ferves as a ful- 

 crum to it ; O is a flender fpring, to keep the lever in 3 

 bearing ftate againft the bar ; and P is a check, to prevent 

 the lever from falling forward, when the bar is taken out ; 

 N is the feeler, fomewhat in the ftiape of a T, fufpended, 

 and moveable lip and down upon the points ot the fcrews, 



1, M, 



