PREFACE. 



EVERYONE who has tried to teach elementary Physics 

 must have become aware of the great difficulty which 

 the subject presents to the majority of pupils. This 

 difficulty is of a twofold kind, and arises partly from 

 the nature of the facts with which the science deals, and 

 partly from the nature of the reasoning, whereby the 

 general laws of physics are established. A large pro- 

 portion of the facts are such as either do not fall within 

 common experience at all, or do so only under such 

 complex conditions that their true nature is not easily 

 recognised ; and moreover the kind of knowledge which 

 is required in physics, is much more accurate and 

 precise than that with which we are accustomed to be 

 satisfied in relation to matters of ordinary life. Hence, 

 in beginning the study of physics, we are obliged, not 

 only to learn a large number of new facts, but also to 

 adopt new habits of learning; while we have, at the 

 same time, to accustom ourselves to attach accurately 

 defined meanings to the terms employed in discussing 

 physical phenomena, and to reason about them with 



