XI 



A CENTURY'S PROGRESS IN PHYSICS 

 By LEIGH PAGE 



DYNAMICS. At the beginning of the nineteenth 

 century mechanics was the only major branch of 

 physical science which had attained any consider- 

 able degree of development. Two centuries earlier, 

 Galileo 's experiments on the rate of fall of iron balls 

 dropped from the top of the Leaning Tower of Pisa, had 

 marked the origin of dynamics. He had easily disproved 

 the prevalent idea that even under conditions where air 

 resistance is negligible heavy bodies would fall more 

 rapidly than light ones, and further experiments had led 

 him to conclude that the increase in velocity is propor- 

 tional to the time elapsed, and not to the distance 

 traversed, as he had at first supposed. Less than a 

 century later Newton had formulated the laws of motion 

 in the same words in which they are given to-day. These 

 laws of motion, coupled with his discovery of the law of 

 universal gravitation, had enabled him to correlate at 

 once^the planetary notions which had proved so puzzling 

 to his predecessors. His success gave a tremendous 

 stimulus to the development and extension of the funda- 

 mental dynamical principles that he had brought to light, 

 which culminated in the work of the great French mathe- 

 maticians, Lagrange and Laplace, a little over a hundred 

 years ago. 



Newton's laws of motion, it must be remembered, 

 apply only to a particle, or to those bodies which can be 

 treated as particles in the problem under consideration. 

 In his 'Mecanique Analytique" Lagrange extended 

 these principles so as to make it possible to treat the 

 motion of a connected system by a method almost as sim- 

 ple as that contained in the second law of motion. 



