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
speed, forever. Contrariwise, a stationary body will remain
forever at rest unless acted on by some disturbing force.

This all-important law, which lies at the very foundation of all
true conceptions of mechanics, was thus worked out during the
first half of the seventeenth century, as the outcome of
numberless experiments for which Galileo's experiments with
failing bodies furnished the foundation. So numerous and so
gradual were the steps by which the reversal of view regarding
moving bodies was effected that it is impossible to trace them in
detail. We must be content to reflect that at the beginning of
the Galilean epoch utterly false notions regarding the subject
were entertained by the very greatest philosophers--by Galileo
himself, for example, and by Kepler--whereas at the close of that
epoch the correct and highly illuminative view had been attained.

We must now consider some other experiments of Galileo which led
to scarcely less-important results. The experiments in question
had to do with the movements of bodies passing down an inclined
plane, and with the allied subject of the motion of a pendulum.
The elaborate experiments of Galileo regarding the former subject
were made by measuring the velocity of a ball rolling down a
plane inclined at various angles. He found that the velocity
acquired by a ball was proportional to the height from which the
ball descended regardless of the steepness of the incline.