ND DEAINAGE. 



different source than that of the gravel 

 beds before mentioned. It will be seen by 

 the diagram, that the strata assume a 

 basin-shaped arrangement, and that be- 

 tween the London clay and the chalk, is 

 the permeable bed of sands, &c. of the 

 plastic clay, this receives the water falling 

 on its surface at b b, and is retained in it 

 by the retentive nature of the beds above 

 and below, and is unable to escape except 

 at the outcrop b, where occasionally over- 

 flowing springs occur when the bed is 

 fully saturated with water. If therefore 

 a well is sunk or a boring made through 

 the London clay into these sands, as shown 

 in the section, water will be reached, and 

 from the tendency to find its level, assisted 

 by the surrounding pressure, it will some- 

 times rise to the surface or within a short 

 distance of it, thus affording a supply of a 

 pure and softer water than from ordinary 

 wells. The fountains in Trafalgar Square, 



e of the breweries and factories in London, are supplied by these wells. Similar Artesian 

 Is may be formed by sinking still deeper into the permeable beds of the upper and lower 

 >n-sand, which outcrop at the surface at c c. The water of the celebrated Artesian well of 

 nelle, at Paris, is derived from these lower cretaceous strata. 



;t simple rules for discovering Springs near the surface. In the early part of the year, if the 

 er is ploughed, if a part be darker than the rest, it may be suspected that water will be 

 3 the ground over the spots where springs are concealed. In all seasons of the year, more 

 i springs, a greater degree of humidity gives rise to more copious exhalations, especially in 

 i when the source is lower, they are rarely sufficient, and the only safe guide is a boring; 

 I of the arrangement of strata in the locality is absolutely necessary. 



F LANDS. 





b, it will be requisite to cut the drains through the retentive soil, and if the porous stratum 

 hallow, through that also. In this case, if a valley exposes as at b the outcrop of the 

 )us bed, the land will be more easily drained; otherwise, if a gully be cut into the porous 

 as at a, the drainage can be carried to lower levels. Where a tongue of porous soil lies 

 n a bed of clay, as shown in the third diagram, producing a swamp or morass, a main drain 

 through the clay at the point D, will be the proper remedy. 



ig. 4. This figure may afford an illustration of unequal drainage, due to the arrangement 

 tie substrata; the land over a will be more effectually drained in consequence of its imme- 

 ely overlying the mixed porous strata, than at b, where it covers the retentive bed; either 

 ace furrows to connect with the part a, or by boring down to M, will render the drainage 

 form. 



'NOLDS, 174, STRAND. 



