SCIENCE-GOSSIP. 



333 



ARTESIAN WELLS: 

 By W. V. Ball. B.A., F.G.S. 



T 1 THEN passing through Trafalgar Square one is 

 often astonished at the immense quantity of 

 water which must daily run to waste from the two 

 beautiful fountains which are constantly playing, 

 and one sometimes comments on the apparently 

 reckless extravagance of the municipal authorities 

 in permitting such waste. The beauty of these 

 two fountains is greatly enhanced by the fact that 

 they are entirely independent of any grimy 

 pumping-engine for that which gives them life. 

 The water, which comes from a depth of 400 

 feet below Charing Cross, is brought to the 

 surface, at considerable pressure, from an Artesian 

 well, and, as will be shortly explained, we have 

 to thank nature alone for the supply to these 

 ornaments to our great city. 



The name " Artesian," as applied to wells, is 

 derived from Artois, one of the French provinces 

 where they exist and are made use of in large 

 numbers. They have been known for a very long 

 time, having been found in China and in the 

 Libyan Desert in the twelfth century. The 

 distinction between an ordinary well and an 

 Artesian well is that whereas the former only 

 brings water to the surface of the ground, and in 

 some cases is only able to bring it to a point some 

 distance below the surface, from which it has to 

 be pumped, the water is brought up by an 

 Artesian well at appreciable pressure, so that it 

 can be conducted in pipes to a considerable height, 

 and be used for supplying fountains, or for ordinary 

 domestic purposes. It is, therefore, obvious that 

 the existence of an Artesian well is due to some 

 very special arrangement of the underlying rocks, 

 and it is only a study of the geological structure 

 of a district that will serve to explain their origin. 



It is well known that different " rocks," in the 

 geological sense, are permeable to water in various 

 degrees. Thus, chalk is not only permeable, but is 

 capable of soaking up large quantities of water like 

 a sponge, while a stiff clay entirely prevents the 

 passage of water through the strata. Consider for 

 a moment a district in which the rocks consist of 

 a basin-shaped series of strata, sloping towards a 

 central point or line. Then, as shown in the 

 accompanying diagram, the newest rocks will be 

 in the centre, and towards the outside the older 

 rocks will come in succession. Suppose that a 

 permeable stratum (c) on the outside of this basin 

 underlies an impermeable stratum (1^) and overlies 

 another impermeable stratum. Where the per- 

 meable stratum comes to the surface, it absorbs 

 the rain-water, which, on penetrating through to 

 the impervious rock underneath, follows the 



general inclination of the basin and runs towards 

 the central point, where it gradually collects. 

 After it has accumulated so as to fill the chalk or 

 other pervious rock to saturation, it is prevented 

 from rising any higher at the central point by the 

 overlying impervious rock (d). In consequence of 

 this the level of the water rises in the pervious rock 

 which forms the sides of the basin, and ultimately 

 rises higher than the level of the bottom of the 

 basin at the surface. Then, as soon as a hole is 

 bored through the bed (d) to the pervious and 

 saturated chalk, the water will rise not only to the 

 surface, but far above it, if pipes are laid to conduct 

 it. The height to which the water will rise is of 

 course regulated by the rainfall, not of the place in 

 which the Artesian well is sunk, but of the districts 

 where the permeable strata crop out at the surface. 

 The theoretical conditions which have just been 

 explained are closely analogous to those which 

 account for the Artesian wells in the London basin, 

 of which Trafalgar Square, roughly speaking, 



Geological Form.\tion for Artesian Well. 



forms the central point. The London clay 

 represents the overlying impervious stratum, and 

 it is through this that the well is bored to a 

 depth of 400 feet. The chalk, which is porous and 

 acts as a sponge, underlies the London clay, and 

 crops out in Hertfordshire to the north and in 

 Surrey to the south of London, while the stiff blue 

 clay known as the gault forms the lowest layer of 

 the basin, and prevents the water from penetrating 

 further into the earth. The pressure of the water 

 in Trafalgar Square therefore varies according as 

 the season is dry or wet in Hertfordshire and Surrey. 

 Thus these fountains, and, indeed, many of the 

 Government buildings at Westminster, are supplied 

 with water, not from any artificial reservoir, but 

 from one which has been constructed, and is 

 constantly replenished, by nature herself. 



Artesian wells are quite common in France, as 

 indeed they always are wherever the geological 

 conditions are favourable. Paris at the present 

 day is to a large extent supplied by them, and the 

 water comes in some cases from so great a depth 

 that it is quite warm when it reaches the surface. 

 This is of interest to geologists, as showing that the 

 temperature of the earth increases with the depth. 

 Cambridge ; April, 1897. 



