285 
the mean density of the earth. 
copper wire ; by any small movement of these balls and the 
connecting rod, in a horizontal direction, by the torsion or 
twisting of the wire, a very minute and slow vibratory motion 
is commenced. To produce this small motion in the two 
little balls, and their connecting rod, two other large balls of 
ten inches diameter, are connected together by certain ma- 
chinery, at like distance as the former, and capable of being 
moved to different distances on the horizontal level with the 
small balls By so setting the large balls near the small 
ones, these are attracted by the former, producing a very 
small motion in them, and in consequence a very slow vibra- 
tion. So minute are these motions, that the extent of the 
vibrations is but a small fraction of an inch, and the duration 
of each vibration is not performed but in the time of several 
minutes, from three or four to near fifteen minutes. So 
minute are these motions, that telescopes and other means 
are necessary to view and to estimate their quantity and 
durations. To produce these minute motions, very complex 
machinery are necessarily employed, while the delicate move- 
ments are watched for many hours together, during many 
days, and recorded with regard to the extent and time of 
each vibration. Then, from these spaces and times, the den- 
sity of the earth is to be calculated, by peculiar theorems, as 
compared with the vibrations of common pendulums that are 
produced by the attractions of the earth. 
All these effects were so minute, and produced by ma- 
chinery so complex, and the results calculated by theorems 
derived from intricate mathematical investigations, that it is 
impossible, at first, for ordinary readers to conceive how any 
accurate results can be deduced from them ; and even for 
