STORAGE AND DISTRIBUTION. 113 



with dry rubble stone and the outer slope sown with 

 grass seed. Sometimes circumstances will prove 

 favourable to such a method of construction, on, of 

 course, a much smaller scale than large waterworks, 

 although the method of construction is precisely the 

 same in both cases. The most important part of the 

 work is the trench. It must be carried down to a 

 firm, hard, and impervious foundation, and it is the 

 duty of the engineer to most rigidly inspect the work 

 when it is opened up to the full depths specified, and 

 to see that all bad places are cut out and filled with 

 clay. The same remarks, of course, apply if the wall 

 was to be of masonry or concrete, as will be de- 

 scribed. Springs encountered must be led away 

 down-stream to a suitable outlet by means of pipes. 

 In firm ground the trench is opened up with sloping 

 sides, but in soft ground the sides would be vertical 

 and the trench timbered. The drawing of the timber 

 when the clay is in demands great care. The clay 

 used must be well worked and quite free from all 

 foreign matter. The thickness of the puddle wall 

 will vary ; no hard and fast rules are given by engin- 

 eers ; the safest way is to make it at the base, that is, 

 the top of the trench, J the depth of the water im- 

 pounded, the top width being about 3 ft. in small 

 reservoirs, and 6 ft. in large ones, and battered ac- 

 cordingly. The clay used must be exposed to the 

 elements as long as possible before use as it is much 

 improved thereby, and the earth bank is carried up 

 at the same time as the puddle wall, when it reaches 

 ground level, while the whole bank should be raised 

 simultaneously from end to end. An example of a 

 clay embankment is given in fig. 85. 



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