GALILEO AND THE NEW PHYSICS 



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 me- 

 chanical principles had entered on a new and prom- 

 ising course. 



Galileo could not demonstrate the retarding in- 

 fluence 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 ex- 

 periment 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 in- 

 herent 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 mo- 

 tion. 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 Philosophic, published in 1644, anv 

 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. 



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