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.