47 s Mr ‘ Cavendish's Experiments to determine 
wire hgh ; by which means it is made strong enough to sup* 
port the balls, though very light. * 
The case is supported, and set horizontal, by four screws, rest- 
ing on posts fixed firmly into the ground : two of them are 
represented in the figure, by S and S ; the two others are not 
represented, to avoid confusion. GG and GG are the end walls 
of the building. W and W are the leaden weights ; which are 
suspended by the copper rods RrPrR, and the wooden bar rr, 
from the centre pin P p. This pin passes through a hole in the 
beam HH, perpendicularly over the centre of the instrument, 
and turns round in it, being prevented from falling by the 
plate p. MM is a pulley, fastened to this pin ; and Mm,a cord 
wound round the pulley, and passing through the end wall ; 
by which the observer may turn it round, and thereby move 
the weights from one situation to the other. 
Fig. 2 . (Tab. XXIV.) is a plan of the instrument. AAAA is the 
case. SSSS, the four screws for supporting it. bb, the arm and 
balls. Wand W, the weights. MM, the pulley for moving them. 
When the weights are in this position, both conspire in drawing 
the arm in the direction h W ; but, when they are removed to the 
situation w and w, represented by the dotted lines, both conspire 
in drawing the arm in the contrary direction b w. These weights 
are prevented from striking the instrument, by pieces of wood, 
which stop them as soon as they come within ~ of an inch of the 
* Mr. Mich ell’s rod was entirely of wood, and was much stronger and stiffer 
than this, though not much heavier ; but, as it had warped when it came to me, I chose 
to make another, and preferred this form, partly as being easier to construct and 
meeting with less resistance from the air, and partly because, from its being of a less 
complicated form, I could more easily compute how much it was attracted by the 
weights. 
