e atmosphere. Later experiments with falling bodies led to the
discovery of laws regarding the accelerated velocity of fall. Such
velocities were found to bear a simple relation to the period of time
from the beginning of the fall. Other experiments, in which balls were
allowed to roll down inclined planes, corroborated the observation that
the pull of gravitation gave a velocity proportionate to the length of
fall, whether such fall were direct or in a slanting direction.

These studies were associated with observations on projectiles,
regarding which Galileo was the first to entertain correct notions.
According to the current idea, a projectile fired, for example, from a
cannon, moved in a straight horizontal line until the propulsive force
was exhausted, and then fell to the ground in a perpendicular line.
Galileo taught that the projectile begins to fall at once on leaving the
mouth of the cannon and traverses a parabolic course. According to his
idea, which is now familiar to every one, a cannon-ball dropped from the
level of the cannon's muzzle will strike the ground simultaneously with
a ball fired horizontally from the cannon. As to the paraboloid course
pursued by the projectile, the resistance of the air is a factor which
Galileo could not accurately compute, and which interferes with the
practical realization of his theory. But this is a minor consideration.
The great importance of his idea consists in the recognition that such
a force as that of gravitation acts in precisely the same way upon all
unsupported bodies, whether or not such bodies be at the same time acted
upon by a force of translation.

Out of these studies of moving bodies was gradually developed a correct
notion of several important general laws of mechanics--laws a knowledge
of which was absolutely essential to the progress of physical science.
The belief in the rotation of the earth made necessary a clear
conception that all bodies at the surface of the earth partake of that
motion quite independently of their various observed motions in relation
to one another. This idea was hard to grasp, as an oft-repeated argument
shows. It was asserted again and again that, if the earth rotates, a
stone dropped from the top of a tower could not fall at the foot of the
tower, since the earth's motion would sweep the tower far away from its
original position while the stone is in transit.

This was one of the stock arguments against the earth's motion,