320 ON THE CONSERVATION OF FORCE. 



and from the ~ connection which it constitutes between 

 natural phenomena of all kinds, even of the remotest 

 times and the most distant places, is especially fitted to 

 give us an idea of what I have described as the character 

 of the natural sciences, which I have chosen as the sub- 

 ject of this lecture. 



This law is the Law of the Conservation of Force, a 

 term the meaning of which I mast first explain. It is not 

 absolutely new ; for individual domains of natural pheno- 

 mena it was enunciated by Newton and Daniel Ber- 

 noulli ; and Rumford and Humphry Davy have recognised 

 distinct features of its presence in the laws of heat. 



The possibility that it was of universal application was 

 first stated by Dr. Julius Robert Mayer, a Schwabian 

 physician (now living in Heilbronn) in the year 1842, 

 while almost simultaneously with, and independently of 

 him, James Prescot Joule, an English manufacturer, made 

 a series of important and difficult experiments on the rela- 

 tion of heat to mechanical force, which supplied the chief 

 points in which the comparison of the new theory with 

 experience was still wanting. 



The law in question asserts, that the quantity of force 

 which can be brought into action in the whole of Nature 

 is unchangeable, and can neither be increased nor di- 

 minished. My first object will be to explain to you what 

 is understood by quantity of force ; or as the same idea 

 is more popularly expressed with reference to its technical 

 application, what we call amount of work in the me- 

 chanical sense of the word. 



The idea of work for machines, or natural processes, is 

 taken from comparison with the working power of man ; 

 and we can therefore best illustrate from human labour, 

 the most important features of the question with which 

 we are concerned. In speaking of the work of machines, 

 and of natural forces, we must, of course, in this compari- 



