MAKING THE DAM. Ill 



When an arrangement is made for the safe overflow of 

 the surplus in some manner, a valve may be attached to 

 the head of the siphon, (d, fig. 44,) by which the flow may 

 be started, or a tap may be fixed to the lower end of the 

 pipe for the same purpose. This would be preferable to 

 the plain siphon, although it would involve the necessity 

 of personal attendance at stated times to discharge the re- 

 servoir. But no one should undertake the irrigation of land 

 who is averse to giving the necessary attention to the de- 

 tails at proper times. An unexpected accident, the work of 

 vermin, the presence of some floating body, or some other 

 trifle, may stop the work, and unless some oversight is 

 given to it, mischief and loss might occur. It is there- 

 fore advisable to depend upon personal effort rather than 

 automatic contrivances, although it may be as well to 

 have the latter in use if it is not made an excuse for 

 neglecting careful supervision. Of all automatic arrange- 

 ments for discharging the water, the balanced trap is 

 the most trustworthy one. 



"Where the surface is not regularly sloping, a hollow or 

 ravine may be made into a pond or reservoir by building 

 a dam across the hollow. In building any dam of this 

 character, the foundation must first be excavated until 

 the solid subsoil is reached, or the dam will leak and its 

 stability be destroyed. A trench at least a fourth of 

 the width of the dam should be dug and filled with 

 puddled earth or clay. . The front and rear of the dam 

 may be made of sods cut from the bottom of the reservoir, 

 and the center up to the top should be made of earth or 

 clay puddled and rammed solidly between the walls of 

 sods. The dam, if a high one, should be at least twice as 

 wide at the bottom as it is high ; and the width of the 

 top should be one-fifth that of the bottom. The 

 inner slope should be 18 inches horizontal to one foot 

 of hight. The bottom of the pond should be made of 

 puddled clay to prevent a waste of water. A section of 



