INTRODUCTION 



original rectilinear planetary and stellar motions do not 

 stand in need of explanation at all ; that motion is just as 

 "natural" to celestial bodies as rest; that the earth ro- 

 tates on its axis because it always did so ; that the sun, 

 moon, earth, planets, stars, comets, asteroids, meteors 

 and what-not move of themselves without either impul- 

 sion or propulsion. If this is seriously to be regarded as 

 a "solution", then every imaginable problem is as good 

 as solved, and fair woman's "just because" rises to the 

 explicitness of a categorical answer. Here is a perti- 

 nent quotation drawn from Dr. Charles A. Young's text- 

 book, General Astronomy, a standard work taught in 

 most of the universities and colleges of this country (Art. 

 400): 



It has been customary with some writers to speak of a body 

 thus moving "uniformly in a straight line" as actuated by a 

 "projectile force", a very unfortunate expression, which is a 

 survival of the Artistotelian idea that rest is more "natural" to 

 matter than motion, and that when a body moves, some force 

 must operate to keep it moving. The mere uniform rectilinear 

 motion of a material mass in empty space implies no action of a 

 physical cause, and demands explanation only as mere existence 

 does. Change of motion, either in speed or direction this alone 

 implies force in operation. 



And here is another, to much the same purpose, from 

 the pen of Prof. Frederick Soddy, the celebrated physi- 

 cist of Glasgow University (Matter and Energy, p. 19) : 



Before the doctrine of its conservation was established, 

 energy was mysterious and unaccountable in its comings and go- 

 ings. To-day it is no longer a mystery. The unaccounted-for 

 appearance or disappearance of a quantity of energy in any 

 process, however complex, would rouse as much scientific interest 

 as the mysterious appearance or disappearance of matter. When 

 it appears it must come from somewhere, and when it disappears 

 it must go somewhere. Gradually this Law of Conservation has 

 supplied the physicist with an experimental test of reality in a 

 changing universe. What appears and disappears mysteriously, 

 giving no clue of its origin or destination, is outside of his 

 province. To him it has no physical existence. What is con- 

 served has physical existence, whether it is tangible and ponder- 

 able like matter, or intangible and imponderable like energy. 

 Early writers, when they really meant what is now called energy, 



