l8o5,] on providing Pure fValer for Londo?i, 49 



system of experiment devised by Dr. Dalton,* to contribute almost 

 nothing to the supply of this subterranean reservoir, in consequence 

 of the great evaporation and the prodigious demands of vegetation 

 during that period. f The speaker pointed out that either of these 

 rivers, the Lea or the Colne, might be regarded as the outflow or 

 yield of a large space of gathering ground, not less than 200 square 

 miles ; and the Colne had this advantage over the Lea, that the 

 valley through which it flowed to the Thames was in perpendicular 

 elevation much superior to that of the Lea. " 



Mr, Dickinson then pointed out that it was only by very long 

 and very definite experience in the actual measurement of the river, 

 that any one could be convinced of the enormous fluctuation in the 

 quantity of its flow, not only according to the season of the year, 

 but between one year and another ; so that the late Mr. Telford, 

 owing to the want of that experience, and to the neglect of seeking 

 information from the best sources, had been led in his survey 

 in 1834 to assume, that he could calculate upon a supply of 32 

 cubic feet per second from one, and that not the most considerable 

 of the branches of the river Colne above referred to, which, at 

 the present time, owing to the drought of last summer and autumn, 

 does not yield mueh above one-third of that quantity. Mr. 

 Dickinson gave it as his opinion, founded on more than forty 

 years* experience, that by taking advantage of the whole supply of 

 the valley, comprehending the four streams which are united at Rick- 

 mansworth, viz. : — the Colne, the Ver (which was the choice 

 of Mr. Telford), the Gade, and the Chess, which latter stream 

 flows past Latimer and Chenies, a supply of 42 cubic feet per 

 second could always be relied upon for London, besides leaving 

 a surplus for the lower part of the valley, which would be aug- 

 mented by the stream from Missenden, which joins the Colne below 

 Harefield ; J and accordingly, he proposed to abstract 42 cubic feet 

 per second at that point, for conveyance to London by a new 

 aqueduct, constructed on more judicious principles than the New 

 Hi ver of the Lea valley. 



A plan of this work, on a very large scale, was exhibited, and 

 the speaker explained various novel contrivances by which he pro- 

 posed to give space for the deposit of every thing of greater specific 

 gravity than water, and to intercept every thing that would float, 

 and to clear away scum, and guard the channel from leaves and 

 vegetable refuse, also to aerate the water ; and, finally, to deliver 

 twenty-three millions of gallons per diem — in other words, twenty- 

 three gallons a piece daily, for a million of individuals — into a 

 reservoir at Kilbum, so filtered, and at the same time so fresh, as to 



* See article " Evaporation," by Dr. Dalton, in " Rees's Cyclopedia." 

 t See a vei'y ingenious course of experiments, published by Mr. Lawes, 

 which shows that every plant of wheat, barley, and beans, takes up in the 

 period of its growth 15lbs. of water from the soil, if the season will afford it. 



X The flow of water at that part, in a full season, index)endent of floods, is 

 not less than four times that amount. 



Vol. IL e 



