22 THE FOUNDATIONS OF SCIENCE 



the extreme case, where the second plane was sup- 

 posed to be made horizontal, no destruction of velo- 

 city would occur, and, friction apart, the body would 

 move straight forward for ever with uniform velocity. 



To appreciate the revolution in science which this 

 conclusion produced, it is necessary to describe shortly 

 the beliefs it displaced. In ordinary life, we are ac- 

 customed to see moving bodies brought to rest by 

 impact with the earth, or by friction, or by other 

 opposing agency ; and this experience led to the idea 

 that motion always required the continuous exertion 

 of some force to maintain it unimpaired. Hence 

 followed in astronomy the invention of vortices to 

 uphold the planetary motions, and in terrestrial 

 dynamics similar confusing conceptions had been 

 introduced. 



At one blow Galileo destroyed all such theories, 

 and cleared the ground for new building. 



But before we deal with those who, in this work, 

 entered into his labours, it is worth pausing to con- 

 sider the improvement in method as well as the 

 actual advance in knowledge effected by Galileo. To 

 study Galileo's researches after those of his pre- 

 decessors is like coming out into the light after 

 groping about in a dark room. In mechanical 

 science, he is the first of the moderns, and his methods 

 are a model of physical investigation for all time. 

 He uses experiment in its right place, and scientific 



