HYDRAULICS. 219 



The maxim, however, that overshot wheels are the 

 more powerful the slower they move, is found in 

 practice to be subject to some limitations. 



Mr SMEATON'S experiments led him to conclude, that 

 overshot-wheels do most work when their circum- 

 ferences move at the rate of 3 feet in a second, and 

 that when they move considerably slower than this, 

 they become unsteady and irregular in their mo- 

 tion. This determination is also to be understood 

 with some latitude. He mentions a wheel 24 feet 

 in diameter, that seemed to produce nearly its full 

 effect, though the circumference moved at the rate 

 of 6 feet in a second ; and another of the diameter 

 of 33 feet, of which the circumference had only a 

 velocity of 2 feet in a second, without any conside- 

 rable loss of power. The first wheel turned round 

 in 12".6, the latter in 51".9, 



SMEATON, Experimental Inquiry concerning the Natu- 

 ral Powers of Water and Wind to turn Mills, &c. 

 Phil Trans, vol. LI. (1759,) p. 100, &c. See par- 

 ticularly p. 133, 134. Dr BREWSTER'S Notes on 

 FERGUSON'S Mechanics. 



Machines moved by the Re-action of Water. 



311. Some machines are moved, not directly by 

 the impulse of a stream of water, but indirectly, 

 by the relief from pressure which the motion of 

 the stream occasions. 



3 The 





