136 PROGRESS OF SCIENCE IN THE CENTURY. 



a part of the shower which would otherwise batter 

 upon it. . . It is necessary also to suppose that par- 

 ticles and masses of matter have a cage-like form, so 

 that enormously more corpuscles pass through them 

 than impinge upon them ; else the gravitation action 

 between two bodies could not be as the product of 

 their masses. '^ * But this speculation is only a pro- 

 visional stop-gap. 



To the easy-going materialists, if any survive, the 

 ignoramus of one of our leading physicists should 

 give pause : — " Directly we use the term ^ weight,' 

 we are confronted with the fact that not yet have we 

 any real clew to that astonishing fact of universal 

 gravitation." f 



Summary. — The foundation of modern physics is 

 in Neivto7is Principia (1687) whose value is more 

 fully appreciated at the end than it was at the begin- 

 ning of the nineteenth century. 



CONSERVATION OF ENERGY. 



The Idea of Energy. — Energy is a convenient term 

 for the power of doing work which is possessed by a 

 material system, or by the ether which modern phys- 

 ics has invented as a hazy background of matter. A 

 stream flowing down a valley illustrates energy of 

 motion, it may turn mill-wheels or bear away bridges ; 

 the reservoir on the plateau illustrates energy of posi- 

 tion, which intention or accident may at any moment 

 bring into operation. These tAVO types of power are, 

 as every one knows, called kinetic energy and poten- 

 tial energy. Whether the kinetic energy be expressed 

 in visible motion, as of the stream, or invisible mo- 



* P. G. Tait, Recent Advances in Physical Science, 1876, 

 pp. 299-300. 



t Prof. Oliver J. Lodge, " Modern Views of Matter," 

 Internal. Monthly, I. (1900), p. 525. 



