lyil] on Water Supply. 107 



usually made in such cases is that by a careful compilation from the 

 rain gauges on or near the gathering ground A, or from gaugings of 

 the stream at the site of the embankment, the amount of water which 

 flowed down the stream and is available for supply is ascertained. 

 If this amount is calculated from the careful statistics of Dr. Mill, or 

 from rain gauges, calculations have to be made as to the amount that 

 is lost to the stream by evaporation or absorption, and even then the 

 calculations have t * be carried further to ascertain the amount which 

 could be obtained by storage in three consecutive dry years. When 

 the amount of storage water has been arrived at in inches of rainfall 

 per annum, it is usual to give one-third of the rainfall in compensa- 

 tion to the stream, and take two-thirds for the town supply. At first 

 sight this seems an unfair bargain, like the lawyer who takes the 

 oyster and gives the client the shell. Here is the town taking the 

 whole of the water and paying the owner by giving him back one- 

 third. But the effect of such a bargain is clearly beneficial to the 

 riparian owners. The promoters have to make a greater reservoir 

 than would be necessary for the town supply in order to store the 

 winter floods, and have to find the capital for that. By storing the 

 winter floods in the reservoir, the riparian owners, who have land 

 liable to flooding, are benefited, and those who use it for power are 

 doubly benefited, for their mills are not drowned for so long a 

 period in winter, and by reason of the compensation water, which, of 

 course, much exceeds the minimum flow of the stream before the 

 construction of the reservoir, they get more worth out of the water in 

 the summer. It means, therefore, that the bargain which has been given 

 effect to in a hundred Acts of Parliament is a fair one after all. But 

 even when the amount of compensation has been determined, there 

 have to be elaborate provisions and arrangements to secure the due 

 discharge of the right number of gallons. The scheme has, even 

 after the compensation has been discharged, to provide for the con- 

 veyance of the water to the area of distribution. That is done by 

 means of a conduit, shown upon the diagram by figures 1, 2, 8, 4, 5. 

 One to two, and five to six is conduit, open cut, or covered cut. 

 When a hill intervenes, it is tunnelled at a gradient, as between 3 and 

 4. and when valleys have to be crossed, the ground which falls below 

 the hydraulic gradient, by pipes 2 to 8 and 4 to 5 under pressure. 

 At 6 the water is delivered by gravitation into a high level service 

 reservoir, thence it is put upon the filter beds, and from there it is 

 drawn into the clear-water tank, and it is from that tank supplied to 

 the town. 



In case of spring waters, these are collected where they issue 

 from the earth from their underground recesses in protected tanks, 

 and from these the w^ater, which, in many cases has been suffi- 

 ciently filtered by Nature, is conveyed to the clear-water tank as 

 in the case of surface water. But when, ' as in the case of surface 

 water, Nature does not produce a ready-made article, and the 



