DRAINAOE. 83 



which is in fact a dam holding up the water in the gravel. 

 In this dam there is a weak place at S, through which water 

 issues permanently (a superficial spring), and runs over the 

 surface from S to 0. This issue has a tendency to lower 

 the water in the gravel to the line Mm. But when con- 

 tinued rains overpower this issue, the water in the gravel 

 rises to the line A a, and meeting with no impediment at 

 the point A, it flows over the surface between A and S. In 

 addition to these more decided outlets, the water is pro- 

 bably constantly squeezing in a slow way through the whole 

 dam. Elkington undertakes to drain the surface from A 

 to 0. He cuts a drain from to B, and then he puts 

 down a borehole, an Artesian well, from B to Z. His hole 

 enters the tail of the gravel ; the water contained therein 

 rises up it; and the tendency of this new outlet is to lover 

 the water to the line B b. If so lowered, it is manifest 

 that it can no longer overflow at A or at S, and the surface 

 from A to is drained so far as the springs are concerned. 

 Though our section can only represent one spring and one 

 summit overflow, it is manifest that, however long the hori- 

 zontal line of junction between the gravel and clay maybe, 

 however numerous the weak places (springs) in the overlap 

 or dam, and the summit overflows, they will all be stopped, 

 provided they lie at a higher level than the line B b. If 

 Elkington had driven his drain forward from B to n, he 

 would at least equally have attained his object ; but the 

 borehole was less expensive. He escapes the deepest and 

 most costly portion of his drain. At x he might have 

 bored to the centre of the earth without ever realising the 

 water in this gravel. His whole success, therefore, depended 

 on his sagacity in hitting the point Z. Another simple 

 and very common case, first successfully treated by Elking- 

 ton, is illustrated by our second drawing, Between grave] 

 hills lies a dish-shaped bed of clay, the gravel being con- 

 tinuous under the dish. Springs overflow at A and B, 

 and wet the surface from A to and from B to 0. D is 

 a drain 4 or 5 feet deep, and having an adequate outlet, 



