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LECTURES TO SCIENCE TEACHERS. 



of rotation ; in fact, if the result were absolute regularity 

 the fly-wheel would be an entirely useless implement. Its 

 object is to store up, when the rotation is in excess of the 

 mean speed, a certain amount of energy in order to deliver 

 it out when the rotation is below that mean speed : there- 

 fore, for it to be of any use, we must presuppose that a 

 certain amount of variation is to be permitted. What we 

 have to see is how much variation we can do with, and 



DIAGRAM 11. 



then see what kind of fly-wheel we ought to employ. Really 

 a, fly-wheel may be likened in its operation to a banker's 

 account. Supposing a man has an income of <! a day, 

 and he spends 1 a day, his income and expenditure 

 are equal and continuous, as he spends every day his income 

 as he gets it ; such a man does not want any balance at 

 his banker's at all. But suppose he spends 1 a day, and 

 gets his income paid to him once in ten days ; if he has not 



