238 IRRIGATION. 



be a very important one. No extensive system of irriga 

 tion can be built up in a year, or a few years. All the 

 great works in existence have been the growths of length 

 ened periods, and have been altered, or improved from 

 time to time. But nevertheless, the construction of a 

 system of irrigation works should not be looked upon as 

 a temporary expedient, that may serve a present purpose, 

 and may be changed as need may arise in the future. 

 This would be a short-sighted policy, and one that would 

 be costly in the end. When works of such a useful 

 character as this are completed, many various interests 

 become involved in their stability, and to change their 

 course or character, might, and undoubtedly would, give 

 rise to damage, disputes, and conflicts. The location of 

 the main works should, therefore, be chosen with every 

 regard to future as well as present requirements. As a 

 general rule, the chief consideration should be to select 

 the location with regard to the most copious supply of 

 water, and the largest amount of territory that may be 

 served by it. The actual supply of water should be ascer 

 tained with great care and exactness, lest costly works 

 may be constructed, and afterwards found to be in 

 adequately provided with water. There are not wanting 

 cases amongst our new works, in which this unfortunate 

 dilemma seems to be inevitable. The fall in the course 

 of the canal determines at least two things ; one, the 

 amount of water which may be passed through it, and 

 the other, the stability of the banks, or the resistance to 

 the wearing action of the current. 



Tfiefall should not be more than one foot in a thousand, 

 except there are the best reasons for departing from these 

 limits. This will give a current of 2 feet per second, or 

 about a mile and a half in an hour. Half of this fall, or 

 2 1 1 2 feet to the mile, may be taken as the standard for 

 average circumstances. This will give a flow of about 

 one mile per hour. 



