418 SUPPLEMENT. 



The upper surface of this machine is thus repre 

 sented: Its frame A B C D is made of mahogany wood, 

 on which is a circle divided into 360 equal parts ; and 

 within that circle is another, divided into 8 equal parts. If 

 the short bar E be pushed one inch forward (or toward the 

 center of 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 45,000dth part of an inch, the index will move 

 one degree of the circle. But as in my pyrometer, the 

 circle is 9 inches in diameter, the motion of the index is 

 visible to half a degree, which answers to the ninety 

 thousandth part of an inch in the motion or pushing of 

 the short 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 

 against the end of the short bar E, over the supporting 

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

 placed close against the end of the short bar, it is plain, 

 that if F expands, it will push E forward, and turn the 

 index e. 



The machine stands on four short pillars, high enough 

 from a table, to let a spirit-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 silver, brass, 

 and iron ; all of the same length as the bar F, for trying 

 experiments on the different expansion of different 

 metals, by equal degrees of heat applied to them for 

 equal lengths of time ; which may be measured by a 

 pendulum, that swings seconds. Thus, 



Put on the brass bar JP, and set the index to the 360th 

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

 count the number of seconds in which the index goes 



