MECHANICS. 419 



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 brass one 

 was before, and then set the index to the 360th degre* 

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

 and let it remain just as many seconds as it did under 

 the brass one ; and then blow it out, and you will see how 

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

 that means you will know in what proportion the ex- 

 pansion of iron is to the expansion of brass ; which I find 

 to be as 210 is to 360, or as 7 is to 12. By this method, 

 the relative expansion of different metals may be found. 



The bars ought to be exactly of equal size; and to 

 have them so, they should be drawn, like wire, through 

 a hole. 



When the lamp is blown out, you will see the index 

 turn backward ; which shews that the metal contracts 

 as it cools. 



The inside of this pyrometer is constructed as follows 



13= 



A a is the short bar, which moves between rollers ; 

 and, on the side a it has 15 teeth in an inch, which 

 take into the leaves of a pinion B (12 in number) on 

 whose axis is the wheel C of 100 teeth, which take into 

 the 10 leaves of the pinion D, on whose axis is the wheel 

 E of 100 teeth, which take into the 10 leaves of the 

 pinion .F, on the top of whose axis is the index above 

 mentioned. 



Now, as the wheels C and E have 100 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 

 2 E 2 



