STORAGE AND REGULATION OF WATER. LIU. 



portion of sand to fill up the voids in the gravel, and 

 briquettes made of 1 of cement to 7 of aggregate. The 

 result was that the 1 to 7 briquette stood 1151bs tensile 

 strain and the stereotyped mixing of 1 to 4 but 45Ibs. after 

 21 days immersion in water in each case. It was also noted 

 that there was an appearance of excess of cement showing 

 on the trowelled surface of the former and insufficient 

 cement in the latter, although the proportions in which 

 the cement w T as actually used were the reverse. 



These large masonry dams, the construction of which 

 has just been discussed, are for the storage of large bodies 

 of water at the heads of oar rivers. The necessity for 

 such works may be taken as an axiom, and it is not possible, 

 without them, to carry out irrigation sections on any but 

 the smallest scale. 



When however, water has been stored it can only be 

 rendered available for irrigation purposes, either by raising 

 its level and so permitting it to flow over the surface of 

 the ground or by pumping it over the banks of the river. 

 The main diversion weir at the head of a canal system 

 must be a fixed structure of sufficient height to turn the 

 required flow down the cuttings under normal conditions. 

 In cases of low river discharge, when little or no water 

 can be spared, the flow down these cuttings must be con- 

 trolled by means of regulating gates. Weirs which are 

 to be used in connection with pumping plants may either 

 be fixed or movable, if the former, they must be of such 

 a height that they will raise the water level sufficiently 

 and also conserve a good supply, and at the same time 

 not so high as to prevent a small flow from passing down 

 the river. To fulfil both of these conditions they should 

 be, say 6 feet high, with a movable crest of " drop boards" 

 by means of which its height may be increased by say, 

 4 feet. A small rise will then fill each storage and pass 



