ARTESIAN WELL 



227 



The Artesian wells in Essex which overflow, are of the following depths, according 

 to Dr. Mitchell : Foulness Island, 450 feet; Mersey, and adjoining islands, 300 feet; 

 Wallis Island, 400 feet; Little Wigborough, 250 feet; Woodham, 350 feet; North 

 Ockenden, 80 ; Fobbing, 100 feet; Bulpham Fen, 70-80. 



The difficulty of making any calculation, even in a well-known district, as to the 

 quantity of water which can be expected to drain to any one borehole, is considerable, 

 since, though it may be possible to judge of the breadth of surface over which the 

 rainfall may bo expected to sink into the water-bearing strata, it is impossible to tell 

 with any degree of exactness the lateral extension of the drainage, except the condi- 

 tions happen to be those in which the borehole is put down in the centre, or some 

 part of a basin. Naturally the more porous and saturablo the wator-bearing strata 

 are, the greater the proportion of the drainage which may be expected to be conveyed 

 to any one point. 



Some years ago Mr. Prestwich computed the quantity of rain falling over the dis- 

 trict surrounding London with a view to estimate the supply of water which a boring 

 through the chalk to the lower greensand formation would furnish. The following 

 Table exhibits the results of his investigation : 



' These calculations, although offered as only very general approximations, give 

 results sufficiently marked and decided, that even admitting the necessity of not incon- 

 siderable corrections, I think they establish strong prim& facie evidence in favour of 

 the upper and lower greensands beneath London containing unusually largo quanti- 

 ties of water, which may be rendered available for the supply of the metropolis by 

 means of Artesian wells. What their yield might be could only bo determined ex- 

 actly by actual experiment ; but, judging from analogy, if the lower tertiary sands, 

 with dimensions comparatively so limited, can nevertheless furnish not less than 

 3,000,000 to 4,000,000 gallons daily (and if, as is probable, they supply much of tha 



3 



