the Density of the Earth. 485 
hour, or an hour, after the motion of the weights ; as it may be 
observed, that in all three experiments, the mean position kept 
increasing for that time, after moving the weights to the posi- 
tive position ; and kept decreasing, after moving them from the 
positive to the midway position. 
The first cause which occurred to me was, that possibly there 
might be a want of elasticity, either in the suspending wire, or 
something it was fastened to, which might make it yield more 
to a given pressure, after a long continuance of that pressure, 
than it did at first. 
To put this to the trial, I moved the index so much, that the 
arm, if not prevented by the sides of the case, would have stood 
at about 50 divisions, so that, as it could not move farther than 
t° 35 divisions, it was kept in a position 15 divisions distant 
from that which it would naturally have assumed from the stiff- 
ness of the wire ; or, in other words, the wire was twisted 1 5 
divisions. After having remained two or three hours in this 
position, the index was moved back, so as to leave the arm at 
liberty to assume its natural position. 
It must be observed, that if a wire is twisted only a little 
more than its elasticity admits of, then, instead of setting, 
as it is called, or acquiring a permanent twist all at once, it 
sets gradually, and, when it is left at liberty, it gradually loses 
part of that set which it acquired ; so that if, in this experiment, 
the wire, by having been kept twisted for two or three hours, 
had gradually yielded to this pressure, or had begun to set, it 
would gradually restore itself, when left at liberty, and the point 
of rest would gradually move backwards ; but, though the ex- 
periment was twice repeated, I could not perceive any such 
effect. 
The arm was next suspended by a stiffer wire. 
