viii Preface 



help him solve his problem. The experiments are made 

 as simple and interesting as possible. They usually re- 

 quire very inexpensive apparatus and are chosen or in- 

 vented both for their interest value and their content 

 value. 



With an explanation of the experiments and the ques- 

 tions that arise, a principle is made clear. Then the 

 pupil is given an opportunity to apply the principle in 

 making intelligible some common fact, if the principle 

 has only intelligence value ; or he is asked to apply the 

 principle to the solution of a practical problem where the 

 principle also has utility value. 



The "inference exercises" which follow each section 

 after the first two consist of statements of well-known 

 facts explainable in terms of some of the principles which 

 precede them. They involve a constant review of the 

 work which has gone before, a review which nevertheless 

 is new work they review the principles by giving them 

 new applications. Furthermore, they give the pupil 

 very definite training in explaining the common things 

 around him. 



For four years a mimeographed edition of this book 

 has been used in the elementary department of the San 

 Francisco State Normal School. During that time va- 

 rious normal students have tried it in public school classes 

 in and around San Francisco and Oakland, and it has 

 recently been used in Winnetka, Illinois. It has been 

 twice revised throughout in response to needs shown by 

 this use. 



The book has proved itself adaptable to either an in- 

 dividual system of instruction or the usual class methods. 



