284 ON THE CONSERVATION OF FORCE. 



any rate, in the present case. The application of this measure 

 is, in fact, not limited to the individual case, but the universal 

 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 

 applicable. 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 expenditure of the next twenty-four 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 she does it abundantly with water, which, being 

 raised to the tops of mountains by meteorological processes, 

 returns in streams from them. The gravity of water we use as 

 moving force, the most direct application being in what are 

 called overshot wheels, one of which is represented in Fig. 38. 

 Along the circumference of such a wheel are a series of buckets, 

 which act as receptacles for the water, and, on the side turned 

 to the observer, have the tops uppermost ; on the opposite side 

 the tops of the buckets are upside-down. The water flows at 

 M into the buckets of the front of the wheel, and at F, where 



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

 it must be multiplied by the intensity of gravity. 



