INTRODUCTION 



The great naturalist Agasslz used to say that a 

 student did not know his subject until he could present 

 it successfully in four different forms : first as a tech- 

 nical monograph, second as a scientific lecture, third 

 as a popular lecture, and fourth as a simple child's 

 tale. Probably the scientific men of our day would 

 flinch or flunk the fourth of these tests. Yet it is 

 possible to put the fundamental fact of any science 

 into a form to be comprehended by the juvenile mind 

 if one knows the knack and takes the trouble to make 

 things plain and interesting. 



And nothing is better worth while, for when the 

 attention of a boy or girl is once directed toward the 

 wonders of nature, and when once he gets the habit 

 of looking for the meaning of what he sees, he has 

 gained an aptitude of mind that will last through life 

 and bring continuously new ideas and inspiration. 



The new views of science that sometimes seem dif- 

 ficult and disconcerting to us elders who have been 

 brought up on the old-fashioned theories, are often 

 clearer and simpler than the old when they are pre- 

 sented directly to the fresh anxi unbiased minds of the 

 younger generation. The modern gasoline motor is 

 easier to understand than the older steam engine, and 

 it is simpler to think of the electric current as a stream 

 of electrons flowing through a wire as water flows 



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