477 
the Density of the Earth . 
The effect of motion in the point of rest is, that when the 
arm is moving in the same direction as the point of rest, the 
time of moving from one extreme point of vibration to the 
other is increased, and it is diminished when they are moving 
in contrary directions ; but, if the point of rest moves uniformly, 
the time of moving from one extreme to the middle point of 
the vibration, will be equal to that of moving from the middle 
point to the other extreme, and moreover, the time of two suc- 
cessive vibrations will be very little altered ; and, therefore, the 
time of moving from the middle point of one vibration to the 
middle point of the next, will also be very little altered. 
It appears, therefore, that on account of the resistance of 
the air, the time at which the arm comes to the middle point of 
the vibration, is not exactly the mean between the times of its 
coming to the extreme points, which causes some inaccuracy 
in my method of finding the time of a vibration. It must be 
observed, however, that as the time of coming to the middle 
point is before the middle of the vibration, both in the first and 
last vibration, and in general is nearly equally so, the error 
produced from this cause must be inconsiderable ; and, on the 
whole, I see no method of finding the time of a vibration which 
is liable to less objection. 
The time of a vibration may be determined, either by pre- 
vious trials, or it may be done at each experiment, by ascer- 
taining the time of the vibrations which the arm is actually put 
into by the fnotion of the weights ; but there is one advantage 
in the latter method, namely, that if there should be any acci- 
dental attraction, such as electricity, in the glass plates through 
which the motion of the arm is seen, which should increase 
the force necessary to draw the arm aside, it would also dimi- 
