HYDRAULICS. 217 



When the undershot-wheel worked to the greatest ad- 

 vantage, its effect was about one-third of the motion 



o 



expended ; we have found it to be . 



27 



Mr SMEATON also found, that the expence of water 

 being the same, the effect is as the effective head, 

 or the square of the velocity. And, lastly, that 

 when the section of the water is the same, the ef- 

 fect is as the cube of the velocity, both which re- 

 sults are deduced above. By a mistake, for which 

 it is difficult to account, SMEATON supposed these 

 conclusions to be inconsistent with the theory of the 

 percussion of fluids. 





Machines moved by the Weight of Water. 



307. When a wheel receives the water into buc- 

 kets, at or near the highest point, it is put in mo- 

 tion by the weight of the water with which it is 

 loaded on one side, and it is then called an overshot 

 wheel. 



308. An overshot wheel arrives at a state of uni- 

 form motion, when the momentum of the water in 

 the buckets is equal to the momentum of the resist- 

 ance. 



The construction of the machine should be such, that 

 when the motion of the wheel becomes uniform, 



the 



