130] The Dialogue on the Two Chief Systems 167 



the supposed motion of the earth. The case of a stone 

 dropped from the top of a tower, which if the earth be 

 in reality moving rapidly from west to east might be 

 expected to fall to the west in its descent, is easily shewn 

 to be exactly parallel to the case of a stone dropped from 

 the mast-head of a ship in motion. The motion towards 

 the east, which the stone when resting on the tower shares 

 with the tower and the earth, is not destroyed in its 

 descent, and it is therefore entirely in accordance with the 

 Coppernican theory that the stone should fall as it does at 

 the foot of the tower.* Similarly, the fact that the clouds, 

 the atmosphere in general, birds flying in it, and loose 

 objects on the surface of the earth, shew no tendency to 

 be left behind as the earth moves rapidly eastward, but 

 are apparently unaffected by the motion of the earch, is 

 shewn to be exactly parallel to the fact that the flies in 

 a ship's cabin and the loose objects there are in no way 

 affected by the uniform onward motion of the ship (though 

 the irregular motions of pitching and rolling do affect them). 

 The stock objection that a cannon-ball shot westward 

 should, on the Coppernican hypothesis, carry farther than 

 one shot eastward under like conditions, is met in the 

 same way ; but it is further pointed out that, owing to 

 the imperfection of gunnery practice, the experiment could 

 not really be tried accurately enough to yield any decisive 

 result. 



The most unsatisfactory part of the Dialogue is the 

 fourth day's discussion, on the tides, of which Galilei 

 suggests with great confidence an explanation based merely 

 on the motion of the earth, while rejecting with scorn the 

 suggestion of Kepler and others correct as far as it 

 went that they were caused by some influence emanating 

 from the moon. It is hardly to be wondered at that the 

 rudimentary mechanical and mathematical knowledge at 

 Galilei's command should not have enabled him to deal 



* Actually, since the top of the tower is describing a slightly larger 

 circle than its foot, the stone is at first moving eastward slightly 

 faster than the foot of the tower, and therefore should reach the 

 ground slightly to the east of it. This displacement is, however, 

 very minute, and can only be detected by more delicate experiments 

 than any devised by Galilei. 



