IV PREFACE 



its neighbors in limitless space, and of the changeless laws 

 that govern its relations with the heavenly bodies. 



The pupil is never plunged into discussions that are beyond 

 his depth. Long, intimate experience with young students 

 has shown how futile it is to presume any background of 

 scientific information on the part of eighth and ninth grade 

 pupils. From the very beginning the book proceeds from 

 the known to the unknown, from the more simple to the less 

 simple. It may be taught in its entirety to immature 

 pupils. 



To make the various subjects more vivid and more in- 

 teresting, practically every topic is illustrated either by a 

 photograph or by a drawing or by both. The many ex- 

 periments help to fix the principles and to inculcate scien- 

 tific habits of thought. 



The present edition contains sixty simple projects which 

 will appeal to boys and girls, and which can easily be 

 worked out without the use of expensive material. 



Thanks are due to the many teachers, especially in Los 

 Angeles, whose suggestions have helped to make the book 

 both teachable and learnable. 



JULY 4, 1919. \y f J S. 



