A HISTORY OF SCIENCE 



distance and then falls to the ground. Plow is this 

 flight of the stone to be explained? The ancient 

 philosophers puzzled more than a little over this 

 problem, and the Aristotelians reached the conclusion 

 that the motion of the hand had imparted a propulsive 

 motion to the air, and that this propulsive motion was 

 transmitted to the stone, pushing it on. Just how 

 the air took on this propulsive property was not ex- 

 plained, and the vagueness of thought that charac- 

 terized the time did not demand an explanation. 

 Possibly the dying away of ripples in water may have 

 furnished, by analogy, an explanation of the gradual 

 dying out of the impulse which propels the stone. 



All of this was, of course, an unfortunate malad- 

 justment of the point of view. As every one nowa- 

 days 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 re- 

 moved, 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 dif- 

 ficult 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 ex- 

 ample, 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 



