804 HORN 



In recapitulating the geological principles on which his scheme is based, Mr. Lucas 

 refers to the obvious tendency of water absorbed by the edges of permeable sediment- 

 ary rocks to sink through these until it readies an impervious stratum, when it flows 

 down the dip of the beds. This is illustrated by the fact that springs which break 

 out at the surface mostly do so from the line of junction of a porous and an impervious 

 bed, and, of course, from the base of the former, especially where the dip is consider- 

 able. In most cases, the drainage of several porous strata is collected at one general 

 point, and to tap this is to obtain, very much more effectually filtered than it can be 

 by any artificial appliances, the water of a largo area of natural drainage. 



Extensive data on this point are given by Mr. Lucas from the experience of miners, 

 to whom these natural incidents are the occasion of great inconvenience and cost. 

 Some of the collieries in the Durham coal-field, sunk through the magnesian limestone, 

 are troubled with as much as 5,000 gallons a minute. So great, however, is the im- 

 perviousness of shale, that five feet of it at the bottom of a shaft forty or fifty fathoms 

 deep through the limestone will turn back that amount of water, allowing none of it to 

 pass through. Mr. Lucas thus concisely indicates the principle of his plan : ' The 

 shaft is sunk to procure a certain stratum ; but were water the object, in this as in all 

 other cases, it would certainly be followed along its bed, at the junction of the sand- 

 stones with the shales, by levels in a manner similar to that in which the ordinary 

 water-levels of a mine are carried along the bed mined for. Nothing could exceed the 

 purity of water that has boon filtered through hundreds of feet of pure quartzosc sand- 

 stone. The springs of clear water that break from the base of sandstones at their 

 outcrop, in all formations, give striking proofs of the truth of this statement. The 

 waters drawn from wells sunk in such beds also corroborate it. If, then, a largo 

 town requires water-supplies, and at no great distance from it there lies a formation 

 containing alternations of porous and impervious beds, which rise above the level of 

 the town, then there should be no difficulty in supplying water at high pressure on the 

 constant system. Such formations occur within easy reach of London.' 



The particular sources from which it is proposed to supply the metropolis are the 

 Lower Greensands of Surrey and the underlying chalk, chiefly the former. The first- 

 named form hills from 800 to 967 feet in height, as at Leith Hill, and Hind Head, and 

 consist of a series of highly porous sands, sandstones, and limestones, 400 feet thick, 

 resting upon a bed of absolutely impervious clay, upon the top of which all the water 

 that sinks through the greensands flows down. The upper part of this latter bed is 

 also sandy, and contains a vast amount of water. This was proved in the construction 

 of the Sevenoaks Tunnel, when the water from the latter beds proved a source of much 

 difficulty to the excavators, and now pours in a fully -formed stream from the tunnel's 

 mouth. ' A tunnel driven along the strike of the beds, or water-level, must of neces- 

 sity arrest all the water that is flowing down it as far as the gallery is carried.' Again, 

 Mr. Lucas argues, ' We have, in fact, in the Lower Greensands of Surrey, a combination 

 of the circumstances most favourable for gathering and storing water. The height of 

 the range attracts a high rainfall, the breadth of the absorbing surface causes much of 

 it to be absorbed. The thickness of the porous beds affords a large reservoir for 

 water absorbed. The complete imperviousness of the retaining clays, Athcrfield and 

 Gault, prevents the escape of these waters, which are thus held in a subterraneous 

 reservoir, whose lowest rim is at the deepest valley cutting through the Gault clays.' 



Mr. Lucas proposes to collect the supply of water for London from the Leith Hill 

 range ' by means of horizontal galleries driven along the strike of the beds on the 

 Atherfield clay, as by that means very large quantities of water may be gained. The 

 height of the range, and the breadth of the absorbing surface presented by it, together 

 with the great thickness of the porous beds, all point to the suitability of the area 

 from a geological point of view. The groat natural reservoir of the chalk of the North 

 Downs of Surrey may also be tapped in a similar manner. The galleries will bo 

 placed in a different position, as the water-bearing part of the chalk is near the top of 

 that formation. As regards actual experience of the Lower Groonsands, the case of the 

 Sevenoaks Tunnel has already been mentioned. That tunnel was driven at right 

 angles to the strike of the beds, and therefore cannot be viewed as a criterion of the 

 quantity of water to be gained by a gallery of equal length carried along the strike, 

 on the Atherfield clay. This quantity would of course bo enormously larger.' See 

 ARTESIAN WEIXS. 



HORN (Eng. and Ger. ; Hois, Come, FT.}, particularly of oxen, cows, goats, and 

 sheep, is a substance soft, tough, semi-transparent, and susceptible of being cut and 

 pressed into a variety of forms ; it is this property that distinguishes it from bono. 

 Turtle or tortoise-shell seems to be of a nature similar to horn, but instead of being 

 of a uniform colour, it is variegated with spots. See TOKT.MSK SHU i.. 



Mr. Aikin ('Trans. Soc. of Arts') remarks, 'In tho Kn^Iish language we have only 

 one word to express two quite different substances ; namely, the branched bony horns 



