47© Mr. Cavendish's Experiments to determine 
8 inches diameter. One of these was to be placed on one side the 
case, opposite to one of the balls, and as near it as could conve- 
niently be done, and the other on the other side, opposite to the 
other ball, so that the attraction of both these weights would con- 
spire in drawing the arm aside; and, when its position, as affected 
by these weights, was ascertained, the weights were to be re- 
moved to the other side of the case, so as to draw the arm the 
contrary way, and the position of the arm was to be again de- 
termined ; and, consequently, half the difference of these posi- 
tions would shew how much the arm was drawn aside by the 
attraction of the weights. 
In order to determine from hence the density of the earth, it 
is necessary to ascertain what force is required to draw the arm 
aside through a given space. This Mr. Michell intended to 
do, by putting the arm in motion, and observing the time of its 
vibrations, from which it may easily be computed.* 
Mr. Michell had prepared tw'o wooden stands, on which 
the leaden weights were to be supported, and pushed forwards, 
till they came almost in contact with the case ; but he seems 
to have intended to move them by hand. 
As the force with which the balls are attracted by these 
weights is excessively minute, .not more than of 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 
* Mr. Coulomb has, in a variety of cases, used a contrivance, of this kind for 
trying small attractions ; but Mr. Michell informed me of his intention of making 
this experiment, and of the method he intended to use, before the publication of any 
of Mr. Coulomb’s experiments. 
