ON THE INTERACTION OF NATURAL FORCES. 157 



reckon the work of steam-engines according to horse- 

 power. The value of manual labour is determined partly 

 by the force which is expended in it (a strong labourer is 

 valued more highly than a weak one), partly, however, 

 by the skill which is brought into action. Skilled work- 

 men are not to be had in any quantity at a moment's 

 notice ; they must have both talent and instruction, their 

 education requires both time and trouble. A machine, 

 on the contrary, which executes work skilfully, can always 

 be multiplied to any extent ; hence its skill has not the 

 high value of human skill in domains where the latter 

 cannot be supplied by machines. Thus the idea of the 

 quantity of work in the case of machines has been limited 

 to the consideration of the expenditure of force ; this was 

 the more important, as indeed most machines are con- 

 structed for the express purpose of exceeding, by the mag- 

 nitude of their effects, the powers of men and animals. 

 Hence, in a mechanical sense, the idea of work has become 

 identical with that of the expenditure of force, and in 

 this way I will apply it in the following pages. 



How, then, can we measure this expenditure, and com- 

 pare it in the case of different machines ? 



I must here conduct you a portion of the way as 

 short a portion as possible over the uninviting field of 

 mathematico-mechanical ideas, in order to bring you to 

 a point of view from which a more rewarding prospect 

 will open. And though the example which I will here 

 choose, namely, that of a water-mill with iron hammer, 

 appears to be tolerably romantic, still, alas ! I must leave 

 the dark forest valley, the foaming brook, the spark- 

 emitting anvil, and the black Cyclops wholly out of sight, 

 and beg a moment's attention for the less poetic side of 

 the question, namely, the machinery. This is driven by a 

 water-wheel, which in its turn is set in motion by the 

 falling water. The axle of the water-wheel has at certain 



