II.] SPRINGS. 33 



demand. Of late years, therefore, most of the deep wells 

 in London have been carried yet lower, passing in fact into 

 the great mass of chalk which underlies the Lower Ter- 

 tiaries in the position represented by C C in Fig. ii. The 

 water is obtained partly from the saturated chalk and partly 

 from fissures, the latter being in this case the more im- 

 portant source. As the position of these irregular cavities 

 cannot be predicted, it is clearly impossible to foretell the 

 depth at which stores of water will be found in the chalk. 



When a deep boring is made through the London clay 

 down to the Lower Tertiary sands, or still deeper, into the 

 chalk, the water tends to rise in the tube, and may even 

 reach the surface and overflow. If the point from which 

 the water is tapped be in low ground, as would be the case 

 with the boring D in the hollow of the London basin, it is 

 necessarily at a much lower level than that of the outcrop 

 of the beds along the margin of the trough, as at C C, 

 The water is consequently forced up the tube by the pres- 

 sure of the liquid with which the water-bearing bed is 

 charged. When this pressure is sufficient to cause tlie 

 water to flow over the surface of the bore-hole, it produces 

 what is termed an "Artesian Well." This name, however, 

 is now commonly extended to other wells in which the 

 water, without overflowing, yet rises to a sufficient height to 

 be economically employed. Artesian wells are of great 

 antiquity in the East ; in North-Westem Europe they were 

 first constructed in the province of Artois, in France, 

 whence the name Artesian. 



In the London basin there are numerous wells supplied 

 on the Artesian principle. Thus the fountains in Trafalgar 

 Square are fed with water from an Artesian well, which 

 penetrates the chalk to a depth of about 390 feet beneath 

 the surface. It would be curious to trace the history of a 



D 



