AUTHOR'S PREFACE. ix 



quences of the law of the Conservation of Force are com- 

 paratively more easy to grasp. They have in recent times 

 been treated by several persons in a vivid and interesting 

 manner, so that it seemed unnecessary to publish the cor- 

 responding part of the cycle of lectures which I delivered 

 on this subject; the more so as some of the more 

 important subjects to be discussed will, perhaps in the 

 immediate future, be capable of more definite treatment 

 than is at present possible. 



On the other hand, I have invariably found that the 

 fundamental ideas of this subject always appear difficult 

 of comprehension not only to those who have not passed 

 through the school of mathematical mechanics ; but even 

 to those who attack the subject with diligence and in- 

 telligence, and who possess a tolerable acquaintance with 

 natural science. It is not to be denied that these ideas 

 are abstractions of a quite peculiar kind. Even such a 

 mind as that of Kant found difficulty in comprehending 

 them; as is shown by his controversy with Leibnitz. 

 Hence I thought it worth while to furnish in a popular 

 form an explanation of these ideas, by referring them to 

 many of the better known mechanical and physical ex- 

 amples ; and therefore I have only for the present given 

 the first Lecture of that series which is devoted to this 

 object. 



The last Lecture was the opening address for the 

 ' Naturforscher-Versammlung,' in Innsbruck. It was not 

 delivered from a complete manuscript, but from brief 

 notes, and was not written out until a year after. The 

 present form has, therefore, no claim to be considered an 



