DISCHARGE FROM RESERVOIRS. 



233 



The hight of the dam above the crest of the waste- 

 weir should differ for different depths of the reservoir. 

 "When the dam is 25 feet high, the waste-weir should be 

 4 feet lower, and for every 25 feet of additional hight of 

 dam, the difference should be increased one foot. The 

 size of the waste-weir should be proportioned to the 

 quantity of overflow to be carried off. This is a matter 

 for calculation of the amount of rainfall and the extent 

 of the area supplying the reservoir. It would always be 

 safe to form a temporary dam of flash boards, or earth, 



Fig. 116. DISCHARGING PLUNGEB VALVE. 



upon the top of the waste-weir, which would raise the 

 surface of the water to the extreme limit of safety, when, 

 if this were overflowed, it would be carried away, and the 

 safe level quickly restored. This is a common practice in 

 India, where a large waste-weir is essentially necessary, 

 on account of the sudden torrents of rain which fall at 

 certain seasons, and where the necessity for saving every 

 gallon of water is paramount. In this way the safety of 

 the works is secured against sudden and unexpected 

 accident. But this possibility of providing for an eventu- 

 ality, which brings danger with it, in this manner, should 



