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XX.— ON THE LIMITS TO THE VELOCITY OF MOTION OF 

 THE WOEKING PAETS OF ENGINES. By GEO. 

 FEAS. FITZGEEALD, F.T.C.D., F.E.S. 



[Read, March 24, 1886.] 



Engines are used for transforming one kind of energy into 

 another. 



Mechanical engines are of two great classes — ones that trans- 

 form potential or statical energy into work, and those that trans- 

 form kinetic energy into work. 



Slow-moving overshot waterwheels may be taken as types of 

 the first class, and windmills as types of the second class. In all 

 cases, it is of course possible by mechanical contrivances, such as 

 levers, pulleys, wheels, &c, to obtain any velocity of moving 

 parts; but the velocity I am calling attention to is the velocity 

 of the parts that move with the working substance. Now, in the 

 case of waterwheels it is evident that when the wheel turns so 

 fast that the water in the buckets is descending as fast as it 

 would fall freely, there can be no work being done by the water 

 on the wheel, and so this limits the rate of working of the wheel. 

 It is to be remarked that in the limiting case the efficiency is 

 zero, while the power is zero when the efficiency is a maximum, 

 i. e. when the wheel is turning most slowly, and that there is a 

 rate of working intermediate between these for which the power 

 is a maximum. In the case of windmills, when the sails turn so 

 fast that the wind blows on unstopped, there is similarly no work 

 being done, and, just as in the other case, this limits their 

 velocity. 



Heat engines are of a different class, as they are for the 

 transformation of irregular into regular motion ; but their 

 mechanical, as distinct from their thermal, arrangements may be 

 grouped as in the last case. . Ordinary steam engines work by 

 means of the energy in the steam doing work by pressing on a 

 piston, and evidently this piston cannot move faster than the 

 steam can follow it up. Professor Osborne Eeynolds has in the 



