WATER AND THE ENGINEER. 661 



As a rule rubble stone may be deposited on the natural bed, 

 but wliere the gi'ound is too soft or treacherous it may be laid on 

 mattresses of mangrove well laced together with wire and pinned 

 down at frequent intervals with piles of small diameter, say, i to 

 6 in. 



The best material for falling immediately behind rubble exposed 

 to wave action is small stones such as quarry chips, or where these 

 are not obtainable good stiff clay or gravelly soil well pinned in 

 layers. 



Tea-tree bark laid between ordinary earth filling and the rubble 

 wall will give the formdr time to settle down hard and firm, before 

 the bark decays. 



The Conservation of Water by Means of Tanks and Dams. 



I have seen many dams constructed by squatters and others that 

 have failed in consequence of defects in location or construction, 

 owing to the employment of the " practical man"' in preference to 

 engineers. Even where entjre failure has not resulted I have 

 frequently observed that the success achieved has not been commen- 

 surate with the outlay. 



The embanking of large streams should never be undertaken 

 except under professional advice and supervision. As a rule, areas 

 exceeding 1,000 acres should be avoided, and, where it is necessary to 

 provide tanks or dams in larger areas, care should be taken to locate 

 the woi-ks to one side of the main watercourse where a sufficient 

 supply of water can safely be led from the principal stream into the 

 tank, so that when it is full the surplus water may safely escape 

 down the main channel. 



Where a dam is so located that a by-wash is required, a sectional 

 area of about 1 square foot to each 3 or 4 acres of catchment area 

 should be provided. 



A very safe method, for those who have no tecluiical knowledge,, 

 is to make a simple hole in level ground in such a position that 

 suiface water can be led into the excavation by means of shallow 

 trenches with but slight fall, so as not to cause much scour. 



Preferably, such excavations should have a depth of at least 

 1*2 ft., so that the inevitable loss by evaporation may not bear too 

 great a proportion to the quantity of water impounded. The loss by 

 evaporation may amount to as much as i ft. or 5 ft. per annum in 

 vertical depth. 



Tlie entrance for stock should be ramped down about 2 to 1,, 

 and roughly pitched with stone, the other sides being protected by 

 fences. The excavated material may be run to spoil in any required 

 direction, provided it is not laid so as to form a continuous embank- 

 ment across the line of drainage. 



Where, as so often occurs in even country, railway embankments 

 are formed from side cuttings, very useful waterholes might be con- 

 structed at but little additional cost, by taking the material from 

 such holes as I have indicated above. 



