THEIR LOCATION. 239 



The fall should be regular from beginning to end, else 

 the current will be more rapid in places where the fall is 

 increased, and this will cause the washing of the banks 

 in the steeper parts, and the deposition of the detritus in 

 places where the current slackens. This, in time, will 

 either destroy the canal or render costly repairs necessary. 

 It may be a question whether it is better to follow a long 

 curve around a hollow, or to carry the canal in a flume or 

 aqueduct directly across it. This question may be decid- 

 ed by considering the cost involved in both plans, and the 

 advantages that may be derived, if any, from adopting the 

 more costly of the two. If there is land that may be 

 conveniently irrigated by following the longer course, that 

 would be a point for consideration. But it should be 

 taken into account that a secondary canal can at any time 

 bs made to supply dependent territory, and it may not be 

 advisable to carry the main canal to it, for no other pur- 

 pose than to supply it. 



The character of the soil in which the bed is to be 

 made, should be regarded in fixing upon the location. 

 There are some canals in existence which lose 40 per cent 

 of their water by filtration through the subsoil. It is 

 evident that, in such cases, it would have been prudent 

 at least to have been sure that no better location could 

 have been selected. 



After this point has been duly settled, the methods of 

 construction need the most careful study. It will be a 

 wise precaution, that may hereafter turn out to be a great 

 economy, to deposit all the excavated earth upon one side 

 only of the canal. If any increase in its size should ever 

 afterwards be determined upon, it can be enlarged at 

 greatly less cost if this precaution is observed. 



The same principles which relate to the construction of 

 the larger canals are also involved in that of the second- 

 ary and distributary ones. The following remarks will 

 therefore apply to irrigating canals of every description, 



