t fall, although joined cohesively to its neighbours. It is
the mass of the attracting body, and not the mass of the attracted
body, that determines the velocity with which the latter moves. The
greater mass of an attracted body expends its superior power, not in
increasing its own rate of motion, but in pulling more energetically
against the attracting mass. Every particle of matter when at rest
resists any attempt to impress it with motion. The amount of this
resistance is called its inertia. When many particles are united
together into one body, they not only, therefore, take to that body
many points upon which the earth's attraction can tell, but they also
carry to it a like quantity of resistance or inertia, which must be
overcome before any given extent of motion can be produced. If the
earth's force be but just able to make particle 1 of any body go
through 200 inches in a second, it will also be but just able to make
particles 2, 3, and 4 do the same; consequently, whether those
particles be separate or combined together, their rate of travelling
will be the same. Hence all bodies descend to the earth with exactly
the same velocities, however different their natures may be in the
matter of weight, always provided there be no retarding influence to
act unequally upon their different bulks and surfaces. It is well
known that even a guinea and feather will fall together when the
atmospheric resistance is removed from their path.

The reader will now, of course, see that what is true of the motion of
free bodies, must also be true of the motion of suspended ones, since
the same terrestrial attraction causes both. There is no reason why
the two-pound weight in the experiment should vibrate quicker than the
one-pound weight, just as there is no reason why a two-ounce bullet
should fall quicker than a one-ounce bullet. Here, also, there are
only the same number of terrestrial particles to act upon each
separate particle of the two unequal weights. Hence it is that the
vibrations of unequal weights are isochronous when hung on strings of
equal lengths.

Thus far our dealings have been with what has seemed to be a very
single-purposed and determined agent. We have hung a weight upon a
piece of string and set it swinging, and have then seen it persisting
in making the same number of beats in the same period of time, whether
we have given it a long journey or a short one to perform; and also
whether we have added to or t