Water Supply. 285 



donkey-wheel, as shown at Carisbrook, are often used ; but for a 

 common well the simpler the machinery the better. Boring from 

 the bottom of a chalk- well is sometimes resorted to with success ; 

 but it must be remembered that the water will not rise higher 

 than the level at which it stands already in the well, though the 

 bore-hole will quicken the supply. It is better, if the well has 

 been made sufficiently deep, to enlarge the bottom in a bell-shape, 

 or if a great quantity of water is required, to drive adits, if there 

 seems no local reason to the contrary, in a direction from the 

 known outfall of the water. 



The upper greensand immediately underlies the chalk or chalk- 

 marl. Its development is very irregular ; in some places it attains 

 the thickness of 140 feet ; in other localities it is scarcely to be 

 traced. Like the chalk, it freely absorbs the water falling on its 

 surface : its water-bed is ruled as that in the chalk, but it is far 

 more accessible ; indeed there is no stratum where water is more 

 regularly distributed or purer in quality. In sinking wells in 

 this stratum care must be taken not to pierce the gault clay below, 

 lest the quality of the water should be affected. The lower green- 

 sand is separated from the upper by the impermeable bed of gault 

 clay : the lower greensand is permeable to water. Though the 

 beds of which it consists vary considerably in their condition and 

 in their capacity of absorption, sometimes a continuous water- 

 level may be traced. It is often necessary to enlarge the bottom 

 of the well to secure sufficient subterranean storage, as the wells, 

 when exhausted, are slowly replenished. Very great care must 

 be taken not, under any circumstances, to pierce the underlying 

 Kimmeridge clay. If the Portland oolite underlies the green- 

 sand, and should the supply from the latter be insufficient, water 

 may be sought in the oolite below, though the water of the green- 

 sand is usually of better and purer quality. When the Portland 

 oolite overlies the Kimmeridge clay, the same caution will apply. 

 Under no circumstances should the surface of the clay be pierced. 

 There is a great similarity in all the oolitic strata as regards the 

 supply of water, yet no strata present more difficulties and ano- 

 malies in the quantities yielded. This is to be attributed to the 

 unequal thickness of the limestone and oolitic seams of rock, 

 interlaced by beds of clay of most variable thickness and power 

 of upholding water. 



The Portland and coralline oolite are limited to a small area, 

 and in many places consist merely of isolated patches, capping 

 the surface of the Kimmeridge or Oxford clay. In such case the 

 spot fixed on may materially influence the supply. In a known 

 case it was desired to obtain water for a farmstead : a well was sunk 

 to suit the convenience of the site, on which the buildings were 

 to be erected ; the water-supply was insufficient. It was pointed 



