TUE IRRIGA TIOX A GE. 49 



'Of the greater efficiency of the inch of water delivered in lagre heads. 

 But even here it is quite as necessary to see at the start how much 

 water you are going to store for so much money. For building a res- 

 ervoir is after all mainly a question of what you can afford to pay for 

 it. If one cannot get water in sufficient heads from the canal it will 

 nearly always pay to make a reservoir large enough for those heads. 

 If it should be so large that it, will be too costly to cement, or line with 

 anything tight, the loss from leaving it unlined will generally be 

 trifling compared with the great advantage of having it. You can 

 generally afford the cost of the excavation and buying more water to 

 offset the loss. I do not mean that it would be wise to buy under such 

 a canal and fortunately we have almost none of the sort here. But if 

 for any reason the water supply is in small continuous flow you can 

 afford to pay considerable to lurn it into large effective heads. 



For city supply considerable expense can be endured to make a 

 reservoir tight that would be too great for an irrigator. Many reser- 

 voirs leak because of weak foundation and the leakage often increases 

 that weakness. To make a foundation perfectly solid with perfectly 

 tight lining makes a reservoir very costly. A thin lining of cement 

 is almost certain to leak under any pressure and quite certain to crack 

 under any considerable pressure. If constantly filled with clear water 

 it is apt to continue to leak and will not puddle the fine openings or 

 the ground beneath it. I cannot find a case in which such a lining 

 has been washed with crude petroleum until it becomes tight. But 

 there is reason to believe that it will not injure the cement. The Ce- 

 ment company's oil tank at Colton has held oil five years without in- 

 jury as far as I can learn to the cement. - Cement mixed with linseed 

 oil makes a very tough and durable combination free from any cracks 

 that the eye caa detect, and is used in Scotland for roofing. Whether 

 it is a true set or whether in the nature of putty, which is linseed oil 

 and whiting, I do not know, but it is certain that the cement is not 

 damaged by the oil. It would be worth while to try cement mixed 

 with crude petroleum into a thin wash so that it would run and lay it 

 over the ordinary cement lining where there is much leakage. Or 

 plain asphaltum could be made to adhere to it if it were first washed 

 with crude petroleum and that allowed to dry in. But in most all 

 cases after a reservoir has had a year or two in which to puddle the 

 loss of leakage, if measurel by a guage while the evaporation is 

 measured at the same time with a floating pan beside it, will be found 

 a mere trifle compared with the benefits of the heads of water it 

 accumulates and which you already understand so well. To make a 

 run of ihirty inches for two days requires storage space of little 

 over one hundred thousand cubic feet or thirty-eight hundred cubic 

 yards. This would be about one hundred feet square and ten feet 

 deep. In many cases the excavation alone will cost so much that you 

 cannot afford to line at all, while in many other cases you can only 



