EFFECT OF ALTERNATE LAYERS OF SAND AND CLAY. 317 



filling the two vallies more, or less with water, and forming wet tracts of 

 country resting upon a lower bed of impervious clay. 



In endeavouring to form a satisfactory opinion as to the best mode of 

 draining a piece of land, it is of great importance to be able to determine 

 not only the immediate natural source of the water we are desirous tore- 

 move, but also the probable quantity it may be necessary to carry off, 

 and the 'permanence of the supply. It is well known, for example, that 

 in many spots, when the accumulated waters are once carried off, there 

 remains only a small and probably intermitting supply, for wjwch an 

 outlet is afterwards to be left and kept open ; while in other localities a 

 constant stream of water is seen to pass along the drains. In connection 

 with this point it is of consequence to make out whether the water is 

 thrown out by surface clays, as in this latter diagram, or flows from 

 among the solid rocks at a greater or less depth — as shown in the prece- 

 ding wood-cuts. That which is thrown out by beds of clay is in most 

 cases derived only from the rains that fall, and is, therefore, liable to in- 

 termit, to cease altogether, or to become more copious, according as the 

 season is dry or otherwise ; while that which escapes from a bed of rock, 

 being independent of the seasons, will seldom vary in quantity. Thus 

 it happens that where surface water only stagnates in the soil of a district, 

 a warm, dry, and long continued summer may cause it to yield a crop 

 of unusual excellence, while other soils fed by springs from beneath may, 

 even in such seasons, still retain moisture enough to render them unfit to 

 rear and ripen a profitable crop of corn. 



7°. There remains one other interesting principle connected with this 

 subject, which I must briefly explain to you. Let C and D in the ac- 



companying wood-cut be two impervious beds through which tjjie water 

 finds noescape, and from which the rains pass oflTonly by the natural in- 

 clination of the ground, and let E be a porous bed from which the water 

 finds a ready escape somewhere towards the right. Then if a boring 

 be sunk through C and D in any part of this tract of country, the wa- 

 ter will descend, and. will be absorbed by the bed E. Such dry, porous 

 or absorbent beds exist in many localities, and the skilful drainer may 

 occasionally avail himself of their aid in easily and effectually freeing 

 land from water, which could not without great cost be permanently 

 drained by any other method. Where water collects on a surface rest- 

 ing upon chalk, or upon the loose sands beneath it, this method of boring 

 is frequently had recourse to in some of our southern counties. One dan- 

 ger, however, is to be guarded against in trying this method, that the 

 bore-rod, namely, may enter a bed which is full of water, and from 

 which, as in Artesian wells, it may readily, and in considerable quantity, 

 ascend. Such a boring it is obvious would only add to the evil, and 

 might render necessary a larger outlay in establishing an efficient syg- 



