112 SMALL WATER SUPPLIES. 



circumstances, but at the best it can only be regarded 

 as a more or less temporary expedient. 



Let us consider the case of a flowing river which, 

 being at such an altitude above the place at which 

 the water will be wanted, that when due allowance 

 has been made for friction in a main of fair size, say 

 6 in., and bends, etc., there will be sufficient pressure 

 on the water to force it to the top of the highest 

 building and with a few feet to spare. It is also well 

 to bear in mind that a water main will get rusty 

 inside as time goes on, and friction will consequently 

 increase, and also the fact that for gravitational pur- 

 poses small mains are not to be advised. The cost 

 of laying a 6-in. main is very little more than a 2-in. 

 main, the difference in cost being merely the cost of 

 the pipes. The extra outlay is well spent, especially 

 when there is not too much " head " to spare. In 

 large reservoirs the construction of such a bank as 

 is being considered is, of course, a work of great 

 magnitude, and such work is not usually found in 

 connexion with estate water supplies. The type of 

 bank referred to is the puddle bank, consisting of a 

 trench dug at right angles to the stream, across, and 

 into the two sides of the valley. At the bottom and 

 sides the trench should penetrate impervious stratum, 

 clay or compact rock without faults. From the 

 bottom of this trench is raised a wall of puddled clay 

 to a height of about 4 ft. or so above top water level. 

 On each side of this bank is tipped earth in layers. 

 All the outer slope is 3 to i and the inner slope 

 2\ to i or 2 to i, as the case may be. The puddle 

 wall is also covered with earth so that it will always 

 retain its moisture, while the inner slope is pitched 



