Vlii PREFACE. 



on the mental discipline to be derived from mastering 

 the reasoning processes by which the general conclu- 

 sions of physics are established, and not on an acquain- 

 tance with, particular physical facts, that teachers are 

 tempted to forget how indispensable a preliminary a 

 knowledge of the facts is to the intelligent study of the 



reasoning. 



The kind of knowledge, however, which is really 

 serviceable for this purpose is not such as can be got 

 by merely reading or hearing descriptions of pheno- 

 mena, or even by seeing experiments made by a 

 teacher : it needs that the student should observe and 

 experiment for himself. It is not merely that the 

 knowledge we obtain, by seeing and handling an object 

 for ourselves, is more vivid and complete than what 

 can be obtained second-hand through the testimony of 

 others, but that a great part of the mental discipline 

 which the study of physics is capable of affording, 

 depends upon our becoming convinced, through direct 

 personal observation, that the general laws of the 

 science represent conclusions truly derived from an 

 accurate examination and comparison of the impressions 

 which the actual phenomena make upon our senses. 

 It is, of course, neither needful nor possible to confine a 

 student's attention exclusively to such matters as he 

 can have personal experience of ; as he advances in his 

 studies he must necessarily depend upon books for the 

 greater part of his knowledge ; but, at the outset of 

 his course, it is very desirable that as far as possible 



