CANALS. 237 



that only a limited supply can be expected. If the source 

 of water is inexhaustible, then only can the wells be made 

 permanent. Thus a few wells in a district, may perhaps 

 yield copiously for a while and then fail, or if the number 

 be added to, the supply of water may be inadequate for 

 all of them, and those at the higher part of the basin 

 will cease to flow the first, and afterwards the remainder 

 will act no longer than the supply holds out. It is certain, 

 therefore, that the risk of expending large sums of money 

 in sinking wells of this kind, will be very great, and that 

 as the number in any locality is increased., the risk of 

 failure increases. Further, the expectation of a perma- 

 nent supply is seen to be delusive, excepting under a nar- 

 row range of circumstances. For these reasons, caution 

 should be exercised in making considerable investments, 

 and founding large hopes upon the basis of irrigating 

 farms by the means of artesian wells. More especially 

 should caution be exercised where an extensive district is 

 to be made dependent upon these wells, and a large 

 number of them are to be sunk in contiguous places. 



C HA P T E K XIX. 



SUPPLY CANALS AND THEIR CONSTRUCTION. 



The proper location of the main supply canals of an 

 irrigation system, is a very important consideration. 

 Upon it depend, in a great measure, the cost of the works 

 and their future efficiency. The first cost of a canal will 

 depend, as a matter of course, upon its length, as well as 

 upon its manner of construction. It may be, in some 

 cases, a matter of little moment whether the course of 

 the canal be in a straight line or should curve with the 

 meanderings of the level, or on the other hand, it may 



