RESERVOIRS AND PONDS. 



97 



feet above the usual level, it is easy to estimate the 

 strength of the rafters required and then double the 

 number of them, putting them no more than two feet 

 apart if the sheeting is of one-inch board doubled. An 

 apron should be put on the dam to receive the overflow. 



FIG. 20— DIVERTING DAM. 



A masonry dam should be built on a foundation of 

 concrete, laid on solid rock over piles driven close to- 

 gether, and both sides protected by sheet piling. The 

 piles should be left to protrude into the concrete foun- 

 dation. Except for waterworks, or an irrigation con- 

 duit, there should be no outlet in the bottom of the 



