was, of course, an unfortunate maladjustment of the point of
view. As every one nowadays knows, the air retards the progress of the
stone, enabling the pull of gravitation to drag it to the earth earlier
than it otherwise could. Were the resistance of the air and the pull of
gravitation removed, the 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 meaning of the first law of motion. It
remained, however, for the great Frenchman Descartes to give precise
expression to this law two years after Galileo's death. As Descartes
expressed it in his Principia Philosophiae, published in 1644, any body
once in motion tends to go on in a straight line, at a uniform rate of