are attracted by these weights is exces- 

 sively minute, not more than 



their weight, it is plain that a very 

 minute disturbing force will be sufficient 

 to destroy the success of the experiment ; 

 and from the following experiments it 

 will appear, that the disturbing force 

 most difficult to guard against is that 

 arising from the variations of heat and 

 cold ; for, if one side of the case is 

 warmer than the other, the air in con- 

 tact with it will be rarefied, and, in 

 consequence, will ascend, while that on 

 the other side will descend, and produce 

 a current which will draw the arm 

 sensibly aside. 



" As I was convinced of the necessity 

 of guarding against this source of error, 

 I resolved to place the apparatus in a room 

 which should remain constantly shut, and 

 to observe the motion of the arm from 

 without, by means of a telescope ; and 

 to suspend the leaden weights in such 

 a manner, that I could move them with- 

 88 



