GENERAL CONSIDERATION. 9 



collected is baled out in order that the work of sinking the smaller 

 tube through the clay may be proceeded with. As soon as this 

 reaches the base of the clay, the water which was under hydrostatic 

 pressure proportional to its depth beneath the level of saturation, 

 gushes into the well which it fills up to that level. At first the water 

 which flows into the well is charged with sand, and thus a cavity 

 is formed in the sandy stratum beneath the lower orifice of the pipe 

 that traverses the clay-bed. This cavity gets enlarged until the 

 area of its internal surface becomes sufficient to allow water to 

 percolate at such a rate as will replenish the well while in use, 

 without the rate of flow being sufficient to disturb the sand, as the 

 percolation is distributed over a sufficiently large area. If the bed 

 of indurated clay is sufficiently thick it will support the weight of 

 the masonry above this cavity without yielding. This explanation, 

 the credit of which is given by Captain Clibborn to Mr. J, S. 

 Beresford, accounts fully for the facts observed, and Captain Clibborn 

 denies that there is anything at all of the nature of artesian action. 

 Yet, as pointed out by Mr. Medlicott in a review of Captain Clibborn's 

 Memoir, 1 the explanation, though fully satisfactory, is not incom- 

 patible with artesian action, so long as this is described as that of 

 water with a greater tendency to flow along the planes of bedding 

 than in a direction at right angles. Yet, it must be admitted that 

 they differ but very little from an ordinary percolation well, their 

 typical feature consisting in the fact that advantage is taken of cer- 

 tain natural conditions to obtain a well of moderate diameter in a 

 permeable medium of very loose material without endangering the 

 stability of the masonry and without any risk of silting. As pointed 

 out by Captain Clibborn, an ordinary percolation well may be sunk 

 in any sand however coarse and unconsolidated, if only its diameter 

 be made large enough. He has also shown that where the clay 

 strata do not exist, they could, leaving aside the question of expen- 

 diture, be replaced artificially by building the well upon a platform 



' Rec. Geol. Surv. lnd., Vol. XVI, pp, 205-209. 



( 9 ) 



