ON THE CONSERVATION OF FORCE. 325 



resistances as on the first, and will therefore do twice as 

 much work as when it can only run down five feet. The 

 weight being the same, the work increases as the height 

 of fall. Hence, we may take the product of the weight 

 into the height of fall as a measure of work, at any rate, 

 in the present case. The application of this measure is, 

 in fact, not limited to the individual case, but the uni- 

 versal standard adopted in manufactures for measuring 

 magnitude of work is 'a. foot 'pound — that is, the amount 

 of work which a pound raised through a foot can produce.^ 



We may apply this measure of work to all kinds of 

 machines, for we should be able to set them all in 

 motion by means of a weight sufficient to turn a pulley. 

 We could thus always express the magnitude of any 

 driving force, for any given machine, by the magnitude 

 and height of fall of such a weight as would be necessary 

 to keep the machine going with its arrangements until it 

 had performed a certain work. Hence it is that the 

 measurement of work by foot pounds is universally ap- 

 plicable. The use of such a weight as a driving force 

 would not indeed be practically advantageous in those 

 cases in which we were compelled to raise it by the power 

 of our own arm ; it would in that case be simpler to work 

 the machine by the direct action of the arm. In the 

 clock we use a weight so that we need not stand the whole 

 day at the clockwork, as we should have to do to move it 

 directly. By winding up the clock we accumulate a store 

 of working capacity in it, which is sufficient for the ex- 

 penditure of the next twenty-foiu: hours. 



The case is somewhat different when Nature herself 

 raises the weight, which then works for us. She does not 

 do this with solid bodies, at least not with such regularity 

 as to be utilised ; but slie does it abundantly with water, 



> This is the technical measure of -work; to convert it into scientific 

 measure it must be multiplied by the intensity of gravity. 



