1S50.J 



THE CIVIL EXGIXEER AND ARCHITECT'S JOURNAL. 



265 



the small cylindrical cove containing an enclosed staircase in the 

 Colosseum. Thougli far inferior in dimensions to Air. Ilosking's, and 

 of somewhat lower proportions — viz., 75 feet in diameter, and about 

 70 high, it could hardly he called small, as its diameter would be 

 considerably greater than that of the rotunda in the Bank of Eng- 

 land (o7 feet), and of the new Coal Exchange (60 feet), or, to quote 

 what may seem authority more to the point, the central rotunda in 

 the Museum erected at IJerlin by Scbinkel, which is not more than 

 67 English feet in diameter. No doubt, the rotunda alone would 

 afford very insufficient additional accommodation; but, besides 

 that, there would be an outer ring-shaped gallery, 30 feet wide 

 and about 25 feet high, which would form room for a considerable 

 collection of works of sculpture on each side of it. In order to 

 avoid interfering with, or coming into contact with the buildings 

 on the north and east sides of the quadrangle, spacious semi- 

 circular recesses or tribunes are substituted for the oblong 

 branches of the plan, which, on the two other sides, would connect 

 the new building with the present ones. Those two tribunes 

 Blight be covered with semi-domes, and the light might be ad- 

 mitted from above, either through the vertex of the concavity of 

 the dome, or through a large arched window in the lunette or 

 arched head of the wall forming the chord of the semicircle. In 

 the western branch from the rotunda would be flights of steps, as 

 in the southern one, in order to ascend to the level of the west 

 gallery, which would there be entered, as in Mr. Hosking's 

 plan, immediately opposite the Elgin Room; but with this differ- 

 ence, that the gallery itself would remain untouched, as all the 

 alteration there required would e.\tend to little more than breaking 

 through t'je wall and forming a door bciient/i the centre window on 

 that side; the windows being 12 feet from the floor, and the con- 

 necting vestibule between that west gallery and the circular one 

 being somewhat lower than the latter, so that its roof (a lead flat) 

 would clear the sill of the window, — as is now done in the new por- 

 tion of that west gallery, where two doors occur beneath windows. 



Such descent and ascent would, it is conceived, he not inappro- 

 priate, as marking transition and creating variety; but whether de- 

 sirable or not on that account, great positive inconvenience would 

 be avoided by placing the rotunda about ten feet below the level of 

 the present ground-floor galleries; for, assuming about tliirty feel 

 as the height of the cylindrical walls, they would rise only eight 

 feet higher than the sills of the windovvs around the quadrangle, 

 so that they would not be visible at all throiigh the windows from 

 within, while the dome, being so much smaller, and consequently 

 further off than in Mr. Hosking's plan, would occasion hardly any 

 difference at all as to light. It may further be remarked, that 

 although it would be no more than what attention to proportion 

 would require, the height of Mr. H.'s rotunda would be somewhat 

 preposterous as far as purpose was concerned, excessive height 

 not being particularly desirable in a hall or gallery intended for 

 the exhibition of sculpture, since sculpture can be placed only on 

 the floor, or against the lower part of the walls. And besides that, 

 in a room 120 feet high, even large statues would show too much 

 like mere ornamental accessories to the architecture, so lofty and 

 spacious a room brought into immediate propinquity with the 

 present galleries could not fail to dwarf them, and cause them to 

 appear diminutive in comparison with it. 



Should the plan here shown, and w hat we have said on the subject, 

 draw forth remarks from others, either in the shape of objections 

 or otherwise, we may have occasion to say something further; but 

 at present we will only add, that the plan is not to be looked upon 

 as a matured one, but merely as a general shajiing out of what 

 might be adopted, with such modification as might be held most 

 e.xpedient. Perhaps it would be an improvement to make the 

 inner rotunda still smaller — to reduce its diameter to about 60 feet, 

 and lower it in the same proportion, so as to admit of obtaining 

 two circular galleries around it, about 25 feet wide each, instead of 

 a single one of 30 feet in width; that would augment accommoda- 

 tion for exhibiting sculpture, with very little extension of plan, 

 and, at the same time, the height of the buildings would not require 

 to be so high by several feet, which, under the peculiar circum- 

 stances of the locale, is a most important consideration. 



Architectural Exhibition.— At the meeting of the Architectural 

 Association, on the 26th, a subscription was handed in by a memher, from 

 "An Architect." who " regrets to tind the sister art to P.iinting and Sculp- 

 ture likely to lie driven, by the unkindness of her relatives, from the portals 

 of the Rujal Academy ; and in the hope, ere long, that all may share a 

 baildiog fully adequate to the requisitions of the three." 



ON THE SUPPLY OF SPRING WATER FOR THE 

 METROPOLIS.* 



On the Geological Conditions which determine the Relative Value of 

 the Water-lmiring Strata of the Tertiary and Cretaceous Series, and 

 on the prol)iil)i/ity of finding in the lower jnenihers of the latter, beneath 

 London, fresh and large Sources of Water Supply, tending possibly to 

 rise to heights considerably above the lower levels of the Metropolis.^ 

 By Joseph Prestwich, jun., Esq., F.G.S. 



The following observations are the result of inquiries connected 

 solely with questions of pure geology. As they have, however, 

 some bearing on a subject which is now of considerable practical 

 importance, it has led me to give them an application which was 

 not anticipated at the time they were made, ami consequently, 

 some of the data more immediately connected with the practical 

 part of the question are wanting; still, the theoretical deductions 

 may possibly be of some use. As the subject is one of great extent, 

 I cannot, in this communication, give more than a general sketch 

 of it. Much of the detail must necessarily be omitted. 



The wells of London derive their supplies of water from three 

 sources: 1st. From the gravel reposing upon the London clay; 

 2ndly. From the beds of sand and clay between the London clay 

 and tlie chalk, which, in the course of the paper, will be designated 

 as the Lower Tertiary Strata (the term "Plastic Clay Formation" 

 being objectionable); and 3rdly. From the chalk. 



VVhether the beds below the chalk may not be available as 

 sources of water sujiply to London, is one of the objects of this 

 inquiry. The first of these sources is small, and does not come 

 within the range of the present subject. The second has led from 

 time to time to the hope that it would prove large and abundant; 

 but from its failing, in many instances, the third has, of late 

 especially, been much resorted to, but with variable success. 



So far as London is concerned, the results yielded by the Lower 

 Tertiary Strata and by the chalk are well known, and might be 

 exactly determined. But in the event of its becoming necessary 

 to inquire into the capability of any other deposit relatively to 

 these two, it is desirable to ascertain the physical conditions under 

 which the present supply is obtained. These are essentially 

 geological, and have not yet been investigated. They are regulated — 



1st. By the litliological character and by the thickness of the 

 water-bearing strata, upon which depends the facility with which 

 water can percolate through them, and their capacity, as a reservoir 

 for water. 



2ndly. By the areas over which the exposed surfaces and outcrops 

 of these strata respectively extend, whereby the quantity of rain 

 water, which they can receive directly, is determined. 



3rdly. By the position which their outcrop occupies on the 

 surface of the country, which governs the amount of surface water 

 that may be absorbed, and also facilitates or prevents their receiving 

 the drainage from off any of the adjacent lands. 



•ithly. By the exposed surfaces being bare, or covered by any 

 form of drift. 



5thly. By the existence of any lines of disturbance, by which 

 the subterranean circulation of the water may be interfered with 

 or stopped. 



Al'ith regard to the first point, the lower tertiary strata vary so 

 much in their thickness, and in their litliological character, that it 

 is necessary to examine them over a wide range, in order to obtain 

 any exact results, otherwise very different conclusions as to their 

 probable water value might be arrived at. 



They consist throughout of a very variable series of sands, 

 pebbles, and mottled clays. The clays usually form distinct and 

 separate beds, and do not blend with the sands; and, as they are 

 not permeable, they must, in estimating the capacity of this series 

 for water, be separated from the sands, which alone will represent 

 the vvater capacity of the series. In the eastern portion of the 

 tertiary district, the beds between the London clay and the chalk 

 are largely developed, and sands predominate. The thickness of 

 the series in the north-east of Kent averages about 120 feet, of 

 which 90 to 100 feet may be sands, and the rest clays. In the 

 north-west of Kent we find a total thickness of from 90 to 100 



• ReaU at tlip Royal Institute of Britibii Arcliitects, July «Ui. 1K50, 

 + Since the (ollowing P.iper was r.rjtlen, Ui« valuable "Report uf ttie General Board 

 ot Healtli on the Supply of Walii- to the Metropolis," lias been published. Had I been 

 aware last aiilinnn, when my arrtntion was more particularly directed to this subject, by 

 the want ol geological inforruation which seemed not unfrequently tj ■ zist in the discus- 

 sions of the many plans relative to different modes of supply, tl-;al such an investigation 

 was in progress, I probably sliould not have thought of engaugiiig upon ttiis inquiry. As 

 it liowever relates to geoloijical principles of general application, as weli as to a specific 

 braucli of the question, neither of which are, I find, touched upoii in the " Report," I 

 venture to submit to the ** Institution" a short statement o) the principal results I hav« 

 arrived at. 



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