stone as projected from the hand would fly on in a straight line,
at an unchanged velocity, forever. But this fact, which is
expressed in what we now term the first law of motion, was
extremely difficult to grasp. The first important step towards it
was perhaps implied in Galileo's study of falling bodies. These
studies, as we have seen, demonstrated that a half-pound weight
and a hundred-pound weight fall with the same velocity. It is,
however, matter of common experience that certain bodies, as, for
example, feathers, do not fall at the same rate of speed with
these heavier bodies. This anomaly demands an explanation, and
the explanation is found in the resistance offered the relatively
light object by the air. Once the idea that the air may thus act
as an impeding force was grasped, the investigator of mechanical
principles had entered on a new and promising course.

Galileo could not demonstrate the retarding influence of air in
the way which became familiar a generation or two later; he could
not put a feather and a coin in a vacuum tube and prove that the
two would there fall with equal velocity, because, in his day,
the air-pump had not yet been invented. The experiment was made
only a generation after the time of Galileo, as we shall see;
but, meantime, the great Italian had fully grasped the idea that
atmospheric resistance plays a most important part in regard to
the motion of falling and projected bodies. Thanks largely to his
own experiments, but partly also to the efforts of others, he had
come, before the end of his life, pretty definitely to realize
that the motion of a projectile, for example, must be thought of
as inherent in the projectile itself, and that the retardation or
ultimate cessation of that motion is due to the action of
antagonistic forces. In other words, he had come to grasp the