47i 
the Density of the Earth. 
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 contact 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 sen- 
sibly 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 sus- 
pend the leaden weights in such manner, that I could move them 
without entering into the room. This difference in the man- 
ner of observing, rendered it necessary to make some alteration 
in Mr. Michell’s apparatus ; and, as there were some parts of 
it which I thought not so convenient as could be wished, I 
chose to make the greatest part of it afresh. 
Fig. i. (Tab. XXIII.) is a longitudinal vertical section 
through the instrument, and the building in which it is placed. 
ABCDDCBAEFFE, is the case ; x and x are the two balls, 
which are suspended by the wires hx from the arm gbmb, 
which is itself suspended by the slender wire gl. This arm 
consists of a slender deal rod hmh , strengthened by a silver 
* M. Cassini, in observing the variation compass placed by him in the Observatory, 
(which was constructed so as to make very minute changes of position visible, and 
in which the needle was suspended by a silk thread,) found that standing near the box, 
in order to observe, drew the needle sensibly aside ; which I have no doubt was caused 
by this current of air. It must be observed, that his compass-box was of metal, which 
transmits heat faster than wood, and also was many inches deep ; both which causes 
served to increase the current of air. To diminish the effect of this current, it is by 
all means advisable to make the box, in which the needle plays, nor much deeper than 
is necessary to prevent the needle from striking against the top and bottom. 
