Still only a part of this quantity appears in the form of surface-water, for a considerable portion of it is absorbed by the ground on which it falls—especially in dry weather—serving either to “lay the dust,” or to convert it into mud. Due regard, therefore, being had to all these considerations, we cannot, consistently with that caution which is necessary in all statistical inquiries, estimate the surface-water of the London streets at more than one thousand millions of gallons per annum, or twenty times the daily mechanical supply to the houses of the entire metropolis, and which it has been asserted is sufficient to exhaust a lake covering the area of St. James’s-park, 30 inches in depth.
The quantity of water annually poured upon the streets in the process of what is termed “watering” amounts, according to the returns of the Board of Health, to 275,000,000 gallons per annum! But as this seldom or never assumes the form of street surface-water, it need form no part of the present estimate.
What proportion of the thousand million gallons of “slop dirt” produced annually in the London streets is carried off down the drains, and what proportion is ladled up by the scavengers, I have no means of ascertaining, but that vast quantities run away into the sewers and there form large deposits of mud, everything tends to prove.
Mr. Lovick, on being asked, “How many loads of deposit have been removed in any one week in the Surrey and Kent district? What is the total quantity of deposit removed in any one week in the whole of the metropolitan district?” replied:
“It is difficult, if not impossible, to ascertain correctly the quantity removed, owing to the variety of forms of sewers and the ever-varying forms assumed by the deposit from the action of varying volumes of water; but I have had observations made on the rate of accumulation, from which I have been enabled roughly to approximate it. In one week, in the Surrey and Kent district, about 1000 yards were removed. In one week, in the whole of the metropolitan districts, including the Surrey and Kent district, between 4000 and 5000 yards were removed; but in portions of the districts these operations were not in progress.”
It is not here stated of what the deposit consisted, but there is no doubt that “mac” from the streets formed a great portion of it. Neither is it stated what period of time had sufficed for the accumulation; but it is evident enough that such deposits in the course of a year must be very great.
The street surface-water has been analyzed by Professor Way, and found to yield different constituents according to the different pavements from which it has been discharged. The results are as follows:—
“Examination of Samples of Water from Street Drainage, taken from the Gullies in the Sewers during the rain of 6th May, 1850.
“The waters were all more or less turbid, and some of them gave off very noxious odours, due principally to the escape of sulphuretted hydrogen gas.
“Some of them were alkaline to test-paper, but the majority were neutral.