n8 SCIENCE AND THE HUMAN MIND 



its consequences, and to compare them with the 

 results of experiment. 



A body falling freely moves too fast for easy and 

 accurate observation on its rate of fall, and, to bring 

 the speed within convenient limits, Galileo convinced 

 himself that a body falling down an inclined plane 

 acquired the same velocity as though it had fallen 

 freely through the same vertical height. He then 

 experimented with inclined planes, and found that the 

 results of his measurements agreed with those calcu- 

 lated from his hypothesis. 



But another result, equally important, which had 

 already been grasped by Leonardo, followed from 

 Galileo's investigations with inclined planes. He 

 found that, after running down one plane, a ball will 

 run up another to a height equal to that of its starting- 

 point, whatever be the slope, provided that friction 

 be negligible. The second plane may be made as 

 long as we please, but still, if the final height be the 

 same as the initial height, the ball will reach its end. 

 It is the height that matters ; the speed of the ball 

 is acquired by virtue of its descent and is not destroyed 

 unless the ball rise. And, if the second plane be 

 horizontal, the ball will run along it for ever with 

 uniform velocity, until checked by friction or some 

 other force. 



To appreciate the importance of this result, it is 

 necessary to realise that, before Galileo's day, except 

 by Leonardo and those he influenced, it was assumed 

 that every motion required the continual exertion of 

 some force to maintain it. The planets had to be 

 kept in motion by hypothetical vortices moving 

 through the heaven and carrying them round in their 



