PYROMETER. 



I, M, which, as well as L, H, are fo adjuftcd as to leave 

 the motion free, but witliout fliakc ; Q R is the handle of 

 the feeler, moveable upon a loofe joint at R, fa that laying 

 hold of it at Q, the feeler is moved up and down, without 

 being affcdled by the irregular preflure of the hand ; the 

 extremity, S, of the feeler is alio furnilhed with a bit of 

 protuberant hard metal, to render its ccntaft with the point 

 of the micrometer-fcrew more perfeA ; T is the mierometer- 

 ferew ; V the divided index-plate ; and W a knob for the 

 handle ; the micrometer-fcrew paffes through two lolid 

 fcrewed holes at D and Y ; the piece Y Z is made a little 

 fpringy, and endeavours to pull the Icrew backwards from 

 the hole at D, and confequently keeps the micrometer- 

 fcrew conftantly bearing againft its threads the fame way, 

 and thereby renders the motion thereof perfeftly fteady and 

 gentle ; X is the index, having diviiions upon it, anfwering 

 to the turns of the fcrew. This piece points out the divi- 

 fions of the plate, as the face of the plate points out the 

 divifors upon the index. When the inftrument is ufcd, lay 

 hold of the knob at Q with one hand, and, moving the 

 feeler up and down, with the other move forward the fcrew 

 T, till its point comes in contadt with the feeler ; then will 

 the plate and index, V and X, (hew the turns and parts. 

 The bafis of this inltrument, as well as the other parts of 

 it, is brafs : one end of which is continued of the fame piece 

 at right angles, to the height of three inches and a half; 

 and the other end afts upon the middle of a lever of the fe- 

 cond kind, whofe fulcrum is in the bafis ; and, therefore, 

 the motion of the extremity of the lever is double the dif- 

 ference between the expanilon of the bar and the bafis. 

 Hence, having the length of the lever from its fulcrum to 

 the point of fufpenfion of the feeler, the diftance between 

 the fulcrum and the point of contaft with the bar, the inches 

 and parts that coiTefpond to a certain number of threads of 

 the micrometer, and the number of divifions in the circum- 

 ference of the index-plate ; the fraftion of an inch exprefled 

 by one divifion of the plate may be deduced. Thofe mea- 

 fures are as follow : from the fulcrum of the lever to the 

 feeler, 5.875 inches ; from the fulcrum to the point of eon- 

 taft, 2.895 inches; length of feventy threads of the fcrew, 

 2.455 inches ; and the divifions in the circumference of the 

 index-plate, 100. Hence the value of one divifion will be 

 the ^ /u"e 3 d part of an inch : but if the fcrew be altered 

 one-fourth of one of thefe divifions, when the contaft be- 

 tween the fcrew and feeler.is well adjufted, the difference of 

 contaft will be very perceivable to the fligliteft obferver ; 

 and, confequently, . ^\ ; th part of an inch is perceivable in 

 this inftrument. When the inftrument is made ufe of, it is 

 immerged, together with the bar to be meafured, in a cif- 

 tern of water ; which water, by means of lamps underneath, 

 is made to, receive any intended degree of heat not greater 

 than that of boiling, and thereby communicates the fame 

 degree of heat to the inftrument, the bar, and to a mer- 

 curial thermometer immerged therein, for the purpofe of 

 afcertaining that degree. See ^g. 6, in which A B is the 

 ciftern, C the cover, which, when the inftrument (Jig. 5.) 

 is raifed upon blocks, goes on between the bar E F and the 

 bafis B C ; D a handle to take off the cover, when hot ; 

 E the mercurial thermometer ; F the cock, to let out the 

 water ; and G H a hollow piece of tin, which fupports 

 feven fpirit lamps, which are raifed higher or lower by the 

 fcrews I and K, in order to give the water in the ciftern a 

 proper degree of heat. With this pyrometer Mr. Smeaton 

 performed feveral experiments, which are arranged in a 

 table ; and their refult agrees very well, he obfervcs, with 

 the proportions of expanfions of feveral metals given by 

 Mr. Ellicott. The following table fhews how much a foot 



in length of each metal grows longer, by an increafe of heat 

 correfponding to 180' 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 io,ooodth 

 part of an inch. 



1. White-glafs barometer-tube ... 100. 



2. Martial regulus of antimony . - 130, 



3. Bliftered fteel - . - - . 138 



4. Hard fteel ..... . 147 



5. Iron . . - . - ' - - 151 



6. Bifmuth ... . - - 167 



7. Copper hammered . - . - 204 



8. Copper eight parti, mixed with tin one - 218 



9. Caft brafs - - - - - - 225 



10. Brafs fixteen parts, with tin one . - 229, 



11. Brafs wire ... ... 232 



12. Speculum metal ... . 232 



13. Spelter folder, to'z. brafs two parts, zinc one 247 



14. Fine pewter . - . . - 274 



15. Grain tin - - - . - 298 



16. Soft folder, ii;z. lead two, tin one . . 301 



17. Zinc eight parts, with tin one, a little hammered 323 



18. Lead ....... 344, 



19. Zinc or fpelter .... . 353 



20. Zinc hammered half an inch ^fr foot - - 373 



For a farther account of this inftrument, with its ufe, fee 

 Phil. Tranf. vol. xlviii. art. 79. p. 598, &c. 



Mr. Fergufon has conftrufted and defcribed a pyrometer, 

 which makes the expanfion of metals by heat vifible to the 

 45,ooodth part of an inch. The upper furface of this 

 machine is reprefented hj Jig. 7. Its frame, A B C D, is 

 made of mahogany, on which is a circle divided into tliree 

 hundred and fixty equal parts ; and within that circle is an- 

 other, divided into eight equal parts. If the (hort bar E be 

 pufhed one inch forward (or toward the centre ot the circle,) 

 the index e will be turned 125 times round the circle of 360 

 parts or degrees. As 125 times 360 is 45,000, it is evident, 

 that if the bar E be moved only the 45,ooodth part of an 

 inch, the index will move one degree of the circle. But, 

 as in this pyrometer the circle is nine inches in diameter, 

 the motion of the index is vifible to half a degree, which 

 anfwers to the 9o,ooodth part of an inch in the motion or 

 puftiing of the fhort bar E. 



One end of a long bar of metal F is laid into a hollow 

 place in a piece of iron G, which is fixed to the frame of 

 the machine ; and the other end of this bar is laid againft 

 the end of the ftiort bar E, over the fupporting crofs-bar 

 HI: and, as the end, f, of the long bar is placed clofe 

 againft the end of the ftiort bar, it is plain that if F expands. 

 It will pufti E forward, and turn the index if. 



The machine ftands upon four ftiort pillars, high enough 

 from a table, to let a fpirit lamp be put on the table under 

 the bar F ; and, when that is done, the heat of the flame 

 of the lamp expands the bar, and turns the index. 



There are bars of different metals, as filver, brafs, and 

 iron ; all of the fame length as the bar F, for trying ex- 

 periments on the different expanfion of different metals, by 

 equal degrees of heat auplied to them for equal lengths of 

 time ; which may be meafured by a pendulum that fwings 

 fcconds. Thus, 



Put on the brafs bar F, and fet the index to the 360th 

 degree : then put the lighted lamp under the bar, and 

 count the number of feconds in which the index goes round 

 the plate, from 360 to 360 again ; and then blow out the 

 lamp, and take away the bar. 



This done, put on an iron bar F where the brafs one 



was 



