188 HELMHOLTZ 



for measuring magnitude of work is a foot 'pound that 

 is, the amount of work which a pound raised through a 

 foot can produce. 1 



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 cer- 

 tain 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 accu- 

 mulate 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 meteorolog- 

 ical processes, returns in streams from them. The gravity 

 of water we use as moving force, the most direct applica- 

 tion being in what are called overshot wheels, one of which 

 is represented in FIG. 90. 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 the mouth 

 begins to incline downwards, it flows out. The buckets on 

 the circumference are filled on the side turned to the ob- 

 server, and empty on the other side. Thus the former are 



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

 measure it must be multiplied by the intensity of gravity. 



