P Y KG ME T E R. 



Pyiometer. Dr.Desaguliers substitutes a long thin plate of steel LN, 

 v "-V-'*-' about y'/odths of an inch broad, and filed roughly, so as 

 PLATE t o m ove the first roller H, by rubbing against it. It is 

 CCCCLXXI. spring.ternpered, and is a little convex towards H ; but 

 p"' "' when it is fastened to the rod at N, there is a spring 

 fixed to the lower brass plate, which draws it straight 

 and tight by the end L, in the direction NL. Instead 

 of the cocks there are two pulleys, P, P, placed hori- 

 zontally, whose broad vertical grooves receive and di- 

 rect the steel plate or roller LN, that is substituted for 

 the rack. In place of the watch spring to support the 

 bars, Dr. Desaguliers used a small brass roller, four 

 tenths of an inch in diameter, having its axis horizon- 

 tal. This roller is raised up by a screw Q so as to sup- 

 port any bar of metal at its end N. By means of these 

 alterations, the sticking of the teeth is prevented, and 

 the motion of the index commences at the instant that 

 the heat is applied to the metallic rods. See Desagu- 

 liers' Experimental Philosophy, vol. i. p. 444. 



3. Description of Mr. Ellicott's Pyrometer. 



Ellicott's The pyrometer, as constructed by Mr. Ellicott, is 

 pyrometer, represented in Plate CCCCLXXI. Fig. 5, where AA 

 *"'- -A. is a flat plate of brass screwed down to a thick piece of 

 mahogany. Upon the plate is screwed two pieces of 

 brass, two of which B, B support the flat iron bar C, 

 called the standard bar. The upper part of the third 

 piece of brass D is a circle, about three inches in dia- 

 meter, divided into 360 degrees, and within the circle 

 is a moveable plate d, divided also into 360 degrees, 

 and a small steel index. The bar of metal E, upon 

 which the experiment is to be made, rests upon the 

 sfandard bar C. A bar, two inches and a half long, is 

 fixed to an axis, which turns on two pieces of brass 

 screwed to one of the supports B ; and to the end of 

 the lever is fastened a chain, or silk line, which, after 

 being coiled round a small cylinder, to which the in- 

 dex on the brass circle D is fastened, passes over a 

 pulley, and has a weight hung to the end of it. Upon 

 the axis which carries the lever is a pulley, one-fourth 

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

 is fastened. The other end of the chain is hooked to 

 a, strong spring G, bearing against one end of the me- 

 tallic bar E. There is another lever H, exactly similar 

 to F, but the chain fastened to the pulley on its axis, 

 is hooked to the standard bar. The line fastened to 

 the end of this lever, after being coiled round a cylin- 

 der, to which the moveable plate is fixed, passes over 

 a small pulley, and has a weight hung to the end of 

 it ; or the same line, passing under a pulley, to which 

 the weight is hung, has its other end fastened to the 

 lever F, so that one weight will serve for both levers, 

 as in the figure. 



When the bar E lengthens by heat, it allows the 

 weight to draw the lever F upwards, by its action on 

 the spring G, and by means of the silk line, the index 

 will be at the same time carried forward in the circle. 

 When the bar E contracts, the index will return back a- 

 gain, and the same motion will be communicated to the 

 standard bar. An elongation of the bar E one-twen- 

 tieth of an inch corresponds to 360, or one revolution 

 pf the index, and one degree to the 7200th part of an inch. 

 In using this pyrometer, the bar of metal is laid on 

 the standard bar. The heat of a lamp is first applied to 

 the standard bar E, and its expansion, as indicated on 

 the moveable plate, is marked. The expansion of the 

 bar E, by the heat communicated to it from the stand- 

 ard bar, is also measured, as marked by the brass circle. 

 The instrument is then allowed to stand till the whole 



is thoroughly cold, then removing the bar E, and lay- Pyrometer, 

 ing another bar of any other metal in its place, heat the -** ~Y~- * 

 standard bar to the same degree of heat as before, which 

 is seen by the moveable plate's marking the same degree 

 of expansion. The index will then show the degree of 

 expansion of the second metal as it did of the first, and 

 in the same way the expansions of different metals by 

 the same degree of heat may be exactly estimated. The 

 results obtained by Mr. Ellicott were as follows, the 

 heat beinty the same : 



Steel. 

 56 



Iron. 

 60 



Gold. 

 73 



Copper. 

 89 



Brass. 

 95 



Silver. 

 103 



Lead. 

 149 



See Phil. Trans. 1736, vol. xxxix. p. 297, for the de- 

 scription of this pyrometer, and the same work 1751, 

 vol. xlvii. p. 485, for an account of the experiments. 



4. Description of Graham's Pyrometer. 



That celebrated artist Mr. Graham constructed also Graham's 

 a pyrometer for measuring minute alterations in the pyrometer. 

 length of metallic bars, but unfortunately he has left no 

 description of it behind him. Mr. Smeaton saw the 

 instrument itself at the house of Mr. Short, and he 

 mentions that those alterations were determined by ad- 

 vancing the point of a micrometer screw till it sensibly 

 stopped against the end of the bar to be measured. 

 This screw,being small, and very lightly hung, was ca- 

 pable of agreement with the 3500dth part of an inch. 

 See Phil Trans. 1754, vol. xlviii. p. 598. 



5. Description of Smeaton's Pyrometer. 



This instrument, founded on the same general prin- Smeaton 'a 

 ciple as Graham's pyrometer, is represented in Plate pyrometer. 

 CCCCLXXI. No. II. Fig. 6, which exhibits it with- Fl s- 6 - 

 out the cistern in which it is used. The bar ABCD 

 is the basis of the instrument. It is one, by one and a 

 half inch thick, and is made of brass. It stands 

 edgewise upwards, and one end is continued of the 

 same piece at right angles, to the height of three and 

 a half inches, to be a firm support for the end of the bar 

 under examination, while the other end acts on the mid- 

 dle of a lever of the second kind, whose fulcrum is on the 

 basis ABCD. The motion at the extremity of the lever 

 is therefore double the difference between the expansion 

 of the bar and the basis. The bar EF to be examined 

 lies in two notches, one of which is fixed to the upright 

 standard AB, and the other to the principal lever HI. 

 The end E of the bar EF bears against the point G, a 

 screw of use in examining the micrometer screw. The 

 other end of the bar EF bears against a small spherically 

 protuberant bit of hard metal, fixed at the same height 

 as G in the principal lever HI. An arbor K is fixed in 

 the basis, which receives at each end the points of the 

 screws H, L, on which the lever HL turns as upon a ful- 

 crum. A slender spring O keeps the lever in a bearing 

 state against the bar, and P is a check to prevent the 

 lever from falling forward when the bar is taken out. 

 The top of the lever is furnished with an appendage 

 N called a feeler, in the shape of a T, suspended and 

 moveable up and down on the points of the screws I,M, 

 which, as well as L,H, are so adjusted as to leave the 

 motion free, but without shake. The handle QR of 

 the feeler is moveable in a loose joint at R, so that 

 laying hold of it at Q, the feeler is moved up and 

 down without being affected by the irregular pressure 

 of the hand. The exiremity S of the feeler is also 

 furnished with a bit of protuberant hard metal, to ren- 

 der more perfect its contact with the point of the mi- 

 crometer screw. The divided index plate is shown at 

 V, and W, a knob for the handle. The micrometer 



