280 ON THE CONSERVATION OF FORCE. 



difficult experiments on the relation 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 unchange- 

 able, and can npither be increased nor diminished. 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 mechanical 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 comparison eliminate anything in which 

 activity of intelligence comes into play. The latter is also 

 capable of the hard and intense work of thinking, which tries a 

 man just as muscular exertion does. But whatever of the 

 actions of intelligence is met with in the work of machines, of 

 corn-be is due to the mind of the constructor and cannot be 

 assigned to the instrument at work. 



Now, the external work of man is of the most varied kind 

 as regards the force or ease, the form and rapidity, of the 

 motions used on it, and the kind of work produced. But both 

 the arm of the blacksmith who delivers his powerful blows with 

 the heavy hammer, and that of the violinist who produces the 

 most delicate variations in sound, and the hand of the lace- 

 maker who works with threads so fine that they are on the verge 

 of the invisible, all these acquire the force which moves them 

 in the same manner and by the same organs, namely, the muscles 

 of the arm. An arm the muscles of which are lamed is in- 

 capable of doing any work ; the moving force of the muscle 

 must be at work in it, and these must obey the nerves, which 

 bring to them orders from the brain. That member is then 

 capable of the greatest variety of motions ; it can compel the 

 most varied instruments to execute the most diverse tasks. 



