THE CONQUEST OF NATURE 



chief of these are, first, the muscular energy of 

 man himself and of animals; second, the energy of 

 air in motion; third, the energy of water in motion 

 or at an elevation; and fourth, the molecular and 

 atomic energies stored in coal, wood, and other com- 

 bustible materials. To these we should probably 

 add the energy of radio-active substances a form of 

 energy only recently discovered and not as yet available 

 on a large scale, but which may sometime become so, 

 when new supplies of radio-active materials have been 

 discovered. It will be the object of succeeding chapters 

 to point out the practical ways in which these various 

 stores of energy are drawn upon and made to do work 

 for man's benefit. 



[4*] 



