vili Introduction 



through a pipe than to think of it as a transmitted 

 strain in a hypothetical ether. See how readily the 

 youngsters take to the mechanics of the automobile 

 and the workings of the radio. 



It is the aim of the Young People's Shelf of 

 Science to present modern views of the several 

 sciences in a comprehensive and attractive form. 

 These books are intended to be read rather than to 

 be studied. They are not designed to be tasks, but 

 recreations. They do not follow the conventional 

 classification of the college, but treat of natural and 

 industrial processes from a novel and practical view- 

 point. They show that science is not a remote and 

 recondite study but a part of everyday life, the unseen 

 foundation of all we see and do. 



Edwin E. Slosson. 

 Washington, D. C, 

 July, 1923. 



