Page:Popular Science Monthly Volume 66.djvu/265

Rh The theories of the lever, of the inclined plane and of the screw were familiar to Leonardo.

The ideas of Aristotle as to motion and rest were not physical, but metaphysical. An example will illustrate his mode of reasoning which satisfied the scientific world for something like two thousand years. When a stone is thrown from the hand why does it continue to move for a time, and why does it eventually come to rest? Where is the cause of motion—in the hand?—or in the stone? If in the hand, how can the stone continue to move after it has left the hand? If in the stone, why does it ever come to rest? Aristotle's answer is that ‘a motion is communicated to the air, the successive parts of which urge the stone onward; each portion of the air continues to act for some little time after it has been acted upon, and the motion ceases when it comes to a particle which can not act after it has been acted upon.’ The confusion of this explanation is complete.

The mechanical ideas of Aristotle and of his successors, as to falling bodies, are expressed in these words: ‘That body, is heavier than another which, in an equal bulk, moves downward quicker.’ Transforming the phrase, we may say, that if two bodies, A and B, are of equal bulk but of different weights, then the heavier body will fall the quicker; or, again, if A weighs ten pounds and B one pound, A will fall faster than B. The Aristotelians of Galileo's time further maintained that A would fall exactly ten times faster than B. Galileo's experiments proved that they fell in precisely the same time. Sixteen hundred years earlier Lucretius had come near to the same truth: “For whenever bodies fall through water and thin air they must quicken their descents in proportion to their weights, because the body of water and subtle nature of air can not retard everything to an equal degree; on the other hand, empty void can not offer resistance to anything in any direction at any time, but must continually give way; and for this reason all things must be moved and borne along with equal velocity, though of unequal weights, through the unresisting void.”

While Kepler was determining the empirical laws according to which the planets move in their orbits, Galileo was laying the foundations of the science of mechanics by which, eventually, Newton was to explain why they so move. The foundations of mechanics rest on experiments made by Galileo, at Pisa, on the laws of falling bodies. It was the opinion of the time that heavy bodies fell faster than light ones, and it was a matter of common observation that a square foot of wood reached the ground before a square foot of paper released at the same time. The fact was explained by Galileo as due to the resistance of the air. In a vacuum they would fall at the same time. By