184 HELMHOLTZ 



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 dimin- 

 ished. 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 elimi- 

 nate 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 mus- 

 cular exertion does. But whatever of the actions of intelli- 

 gence is met with in the work of machines, of course 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 power- 

 ful 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 arms. 

 An arm the muscles of which are lamed is incapable 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 capa- 

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

 most varied instruments to execute the most diverse tasks. 



Just so it is with machines: they are used for the most 



