THERMOMETER AND PYROMETER, 



bar, and where it touches the index 20 

 times as long as the space between the 

 point of the bar and its fulcrum ; and 

 the space between the end of the lever 

 and the free end of the index is just 20 

 times the length of that between the 

 point of the lever and the axis of the 

 index; hence, when the bar expands 

 5^ 5 th of an inch, the point of the index 

 will have moved over (20 x 20) = 400 

 times as much space, or one inch ; or 

 if the bar expand 3 oW tn of an inch, the 

 index will move j\jth. 



The scale is divided into inches and 



Fig. 16 



tenths ; and the mere friction of the 

 bar /, which is removable at pleasure, 

 with a piece of flannel till it becomes 

 sensibly warm, will be sufficient to show 

 variations of the index. Ferguson states 

 that it gave the following results: 

 with bars of iron and steel, 3 ; copper, 

 4 ; brass, 5 ; tin, 6 ; lead, 7. 



9. In the supplement to his lectures 

 there is however a much more delicate 

 pyrometer described, (fg. 16,) which 

 will show the expansion of a bar of me- 

 tal to the sskotfth of an inch, or even to 

 the 9 0,0 00th. 



The frame a b is of mahogany, sup- 

 ported on short pillars, so as to admit a 

 lamp under it for heating the bar/"; one 

 end of which lies in a cavity in the piece 

 of metal g, and the other, after passing 

 on a friction wheel over the cross-bar 

 h h, presses against the short lever e e. 



The manner in which this short lever 

 acts on the index is seen in the adjoining 

 diagram A, where k is the short lever 

 that moves under the dial d between 

 friction wheels. On the side of k are 

 1 5 teeth in the space of one inch, which 

 play in the twelve leaves of the pinion 



