RESERVOIRS. 107 



for immediate use, and its value is greatly enhanced by 

 being raised to an equal or greater temperature than that 

 of the air. This is most quickly done by exposure in a 

 shallow pond. Every foot saved in depth is a foot added 

 to the level of the outlet, and so much more added to 

 the area that may be irrigated. This is evident, because 

 if the reservoir is 7 feet deep, the surface of the water 

 can be no higher than the level of the source, unless the 

 water is pumped up into the reservoir, and it is clear 

 that the water discharged cannot be made to irrigate 

 any land that lies higher than the bottom of the res- 

 ervoir. With a 7 foot reservoir, all the land that lies 

 between the levels of the bottom of the reservoir and the 

 surface of the water cannot be irrigated ; unless there are 



Fig. 44. SECTION OF RESERVOIR. 



several discharging pipes at different portions of the 

 reservoir. With regard to cheapness of construction, if 

 not to effectiveness in operation, it will be found far bet- 

 ter to have the reservoir as large as possible, at least of 

 sufficient capacity to contain water enough for use every 

 two to seven days. 



Where the surface slopes but one way, an embankment 

 may be made on three sides of a square, inclosing a 

 sufficient space, and open on the upper side at which the 

 spring will discharge itself. This is shown at fig. 44 in 

 section, and in plan at fig. 45. To irrigate the strip of 

 land parallel with the reservoir, a canal or furrow may 

 be carried on a level with the spring, seen at #, a, in the 

 figures, to the boundary of the meadow. The overflow 

 from the reservoir may be made to pass into this canal. 



