A HISTORY OF SCIENCE 



According to his idea, which is now familiar to every 

 one, a cannon-ball dropped from the level of the 

 cannon's muzzle will strike the ground simultaneously 

 with a ball fired horizontally from the cannon. As to 

 the paraboloid course pursued by the projectile, the 

 resistance of the air is a factor which Galileo could 

 not accurately compute, and which interferes with 

 the practical realization of his theory. But this is a 

 minor consideration. The great importance of his 

 idea consists in the recognition that such a force as 

 that of gravitation acts in precisely the same way 

 upon all unsupported bodies, whether or not such 

 bodies be at the same time acted upon by a force of 

 translation. 



Out of these studies of moving bodies was grad- 

 ually developed a correct notion of several impor- 

 tant general laws of mechanics laws a knowledge 

 of which was absolutely essential to the progress of 

 physical science. The belief in the rotation of the 

 earth made necessary a clear conception that all bodies 

 at the surface of the earth partake of that motion 

 quite independently of their various observed motions 

 in relation to one another. This idea was hard to 

 grasp, as an oft -repeated argument shows. It was 

 asserted again and again that, if the earth rotates, a 

 stone dropped from the top of a tower could not fall 

 at the foot of the tower, since the earth's motion 

 would sweep the tower far away from its original 

 position while the stone is in transit. 



This was one of the stock arguments against the 

 earth's motion, yet it was one that could be refuted 

 with the greatest ease by reasoning from strictly 



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