37 



of water to the metropolis, in support of the practicability of afford- 

 ing a supply of filtered water from the Thames, adequate to the de- 

 mand, and within reasonable limits in point of expense, proposes his 

 plan of forming a filter under the bed of the river for each Company. 

 He states that the deposit of mud on each side of the Thames does 

 not reach below the low-water mark, and that the bed of the river 

 throughout is generally a clean and strong, though porous gravel. 

 The mud, therefore, will puddle in, and close the pores of the bed 

 of gravel on which it lies, above low-water mark, so that the filtra- 

 tion into the neighbouring wells, the waters of which are remarkably 

 pure, must take place below low-water mark. He therefore pro- 

 poses to construct a filtering chamber below the bed of the river, 

 from which chamber a main pipe or tunnel must be made for con- 

 ducting the filtered water into a well on the river side, whence it is 

 to be drawn up by steam power, and distributed to the houses to be 

 supplied, by the mains and branches at present existing. 



The filtering chamber and apparatus are to be prepared by erect- 

 ing a coffer-dam in the river, of sufficient size to inclose the whole 

 of the area required for that purpose. This coffer-dam will require 

 piles of forty-five feet in length. The bed of the river, thus laid 

 dry, is to be dug out, and a bed of brick-work, set in cement, laid 

 down : a floor must then be constructed in the form of an inverted 

 segment of an arch. On the top of the walls of this floor, plates are 

 to be laid, and in the inclosed area, granite blocks placed ; upon 

 these again, the girders are to be laid, and over these the joists, 

 which are to support a first layer of large flints. Upon these, suc- 

 cessive layers of smaller flints are to be laid, each decreasing in size 

 as they approach the bed of the river. Upon the uppermost of 

 these, a stratum of clean shingle is to be deposited ; then a bed of 

 fine and very clean gravel ; and lastly a bed of filtering sand, until 

 it arrives within a foot of the bed of the river, which last space must 

 then be filled up with clean gravel ; thereby forming a filtering bed 

 of eight feet in depth, the top of which will still be four feet below 

 low -water mark. So that, allowing seven feet for the timbers and 

 brick-work below, and eighteen feet for the rise and fall of the tide, 

 the total depth at high-water will be thirty-seven feet. 



The paper is accompanied by a lithographic drawing, which ex- 

 hibits the several parts of the scheme. 



A Paper was read, entitled, " On the Variable Intensity of Ter- 

 restrial Magnetism, and the Influence of the Aurora Borealis upon 

 it." By Robert Were Fox. Communicated by Davies Gilbert, Esq. 

 V.P.U.S. 



The author gives the results of a series of observations on the vi- 

 brations of the magnetic needle, which he undertook last summer, 

 for the purpose of ascertaining whether the intensity of its directive 

 force is affected by the changes in the earth's distance from the sun, 

 or by its declination with respect to the plane of its equator. He 

 observed that the magnetic intensity is subject to frequent variations, 

 which are sometimes sudden, and of short duration. These anoma- 



