FLOODING. 67 



on the pole from the height of the spirit-level above the 

 surface of the ground. 



By way of illustration, let the mark, A, (fig. 26) be one 

 foot high, and the top of the level, B, be three feet above 

 the ground, C. Then will BC, less AD, equal two feet. 

 And it will require a dam two feet high, erected at the 

 point, C, to raise water to the top of the knoll, D. 



Having thus determined the greatest depth of the re- 

 quired pond, build your dam accordingly, making it fully 

 as thicJc as the water will be deep ; for instance, if it is 

 found that a " head " of six feet will be required, dig a 



Fig'. 27. A DAM PROTECTED BY TURF. 



ditch, six feet wide, entirely through the muck, and fill it 

 up with clean sand for the foundation of the dam. On 

 each side of this foundation erect a substantial turf wall, 

 six feet thick, to support and protect the true dam, which 

 is afterward made by filling up and packing with sand 

 the space between the walls. 



The dam should be made somewhat higher than it 

 would generally be required, to prevent freshets from 

 overflowing and washing it away. Fig. 27 represents a 

 dam such as we have described. 



The object of having turf on each side is not to hold 

 water, but to economize the sand by supporting it just 



