Of the Surface-Water of the Streets of London.

The consideration of what Professor Way has called the “street waters” of the metropolis, is one of as great moment as any of those I have previously treated in my details concerning street refuse, whether “mac,” mud, or dung. Indeed, water enters largely into the composition of the two former substances, while even the street dung is greatly affected by the rain.

The feeders of the street, as regards the street surface-water, are principally the rains. I will first consider the amount of surface-water supplied by the rain descending upon the area of the metropolis: upon the roofs of the houses, and the pavement of the streets and roads.

The depth of rain falling in London in the different months, according to the observations and calculations of the most eminent meteorologists, is as follows:—

Months.	Depth of Rain in inches.			Quantity of rain falling in the different seasons.	Number of days on which rain falls.
Months.	Royal Society, according to observation.	Howard, according to observation.	Daniell, according to calculation.	Quantity of rain falling in the different seasons.	Number of days on which rain falls.
January	1·56	1·907	1·483	Winter. 5·868	14·4
February	1·45	1·643	0·746		15·8
March	1·36	1·542	1·440		12·7
April	1·55	1·719	1·786	Spring. 4·813	14·0
May	1·67	2·036	1·853		15·8
June	1·98	1·964	1·830		11·8
July	2·44	2·592	2·516	Summer. 6·682	16·1
August	2·37	2·134	1·453		16·3
September	2·97	1·644	2·193		12·3
October	2·46	2·872	2·073	Autumn. 7·441	16·2
November	2·58	2·637	2·400		15·0
December	1·65	2·489	2·426		17·7
Totals	24·04	25·179	22·199	24·804	178·1

The rainfall in London, according to a ten years’ average of the Royal Society’s observations, amounts to 23 inches; in 1848 it was as high as 28 inches, and in 1847 as low as 15 inches. The depth of rain annually falling near London is stated by Mr. Luke Howard to be, on an average of 23 years (1797-1819), as much as 25·179 inches. Mr. Daniell says that the average annual fall is 23⅒ inches. The mean of the observations made at Greenwich between the years 1838 and 1849 was 24·84 inches.

The following extract from an account of the “Soft Water Springs of the Surrey Sands,” by the Hon. Wm. Napier, is interesting.

“The amount of rainfall,” says the Author, “is taken from a register kept at the Royal Military College, Sandhurst, from the year 1818 to 1846.

“The average fall of the last 15 years, during which time the register appears to have been correctly kept, is 22·64 inches. I consider this to be a very low estimate, however, of the average rainfall over the whole district. The fall on the ranges of the Hindhead must considerably exceed this amount, for I find in White’s ‘Selborne,’ a register for ten years at that place; the greatest fall being in 1782, 50·26 inches, the lowest, in 1788, 22·50 inches, and the average of all 37·58 inches. The elevation of the Hindhead is about 800 feet above mean tide.

“With reference to the measurement of rainfall, it is difficult indeed to obtain more than a very approximate idea for a given district of not very great extent; the method of measurement is so uncertain, as liable to be affected by currents of air and evaporation. It is well known that elevated regions attract by condensation more rain than low lands, and yet a rain-gauge placed on the ground will register a greater fall than one placed immediately, and even at a small height, above it.

“M. Arago has shown from 12 years’ observations at Paris, that the average depth of rain on the terrace of the Observatory was 19·88 inches, while 30 yards lower it was 22·21 inches. Dr. Heberden has shown the rainfall on the top of Westminster Cathedral, during a certain period to be only 12·09 inches, and at a lower level on the top of a house in the neighbourhood to be 22·608 inches. This fact has been observed all over the world, and I can only account for it as arising partly from the greater amount of condensation the nearer the earth’s surface, but probably also from currents of air depriving a rain-gauge at a high elevation of its fair share.”

The results of the above observations, as to the yearly quantity of rain falling in the metropolis, may be summed up as follows:—

	Inches of Rain falling Annually.
Royal Society (average of 20 years)	24·04
Mr. Howard (average of 23 years)	25·179
Professor Daniell	22·199
Dr. Heberden	22·608
Mean	23·506

The “mean mean,” or average of all the averages here given is within a fraction the average of the Royal Society’s Observations for 10 years, and this is the quantity that I shall adopt in my calculations as to the gross volume of rain falling over the entire area of London.

I have shown, by a detail of the respective districts in the Registrar General’s department, that the metropolis contains 74,070 statute acres. Every square inch of this extent, as garden, arable, or pasture ground, or as road or street, or waste place, or house, or inclosed yard or lawn, of course receives its modicum of rain. Each acre comprises 6,272,640 square inches, and we thus find the whole metropolitan area to contain a number of square inches, almost beyond the terms of popular arithmetic, and best expressible in figures.

Area of metropolis in square inches, 464,614,444,800. Now, multiplying these four hundred and sixty four thousand, six hundred and fourteen millions, four hundred and forty-four thousand, eight hundred square inches, by 23, the number of inches of rain falling every year in London, we have the following result:—

Total quantity of rain falling yearly in the metropolis, 10,686,132,230,400 cubic inches.

Then, as a fraction more than 277¼ cubic inches of water represent a weight of 10 lbs., and an admeasurement of a gallon, we have the following further results:—

	Weight in pounds and tons.	Admeasurement in gallons.
Yearly Rainfall in the Metropolis	385,399,721,220 lbs., or 172,053,447 tons.	38,539,972,122 gals.

The total quantity of water mechanically supplied every day to the metropolis is said to be in round numbers 55,000,000 gallons, the amount being made up in the following manner:—

Daily Mechanical Supply of Water to Metropolis.

Sources of Supply.	Average No. of Gallons per day.
New River	14,149,315
East London	8,829,462
Chelsea	3,940,730
West Middlesex	3,334,054
Grand Junction	3,532,013
Lambeth	3,077,260
Southwark and Vauxhall	6,313,716
Kent	1,079,311
Hampstead	427,468
Total from Companies		44,383,329
Artesian Wells		8,000,000
Land Spring Pumps		3,000,000
Total daily		55,383,329

Yearly Mechanical Supply of Water.

From Companies		16,200,000,000	gals.
„	Artesian Wells	1,920,000,000	„
„	Land Spring Pumps	1,095,000,000	„
Total yearly		19,215,000,000	„

Hence it would appear that the rain falling in London in the course of the year is rather more than double that of the entire quantity of water annually supplied to the metropolis by mechanical means, the rain-water being to the other as 2·005 to 1·000.

Now, in order to ascertain what proportion of the entire volume of rain comes under the denomination of street surface-water, we must first deduct from the gross quantity falling the amount said to be caught, and which, in contradistinction to that mechanically supplied to the houses of the metropolis is termed, “catch.” This is estimated at 1,000,000 gallons per diem, or 365,000,000 gallons yearly.

But we must also subtract from the gross quantity of rain-water that which falls on the roofs as well as on the “back premises” and yards of houses, and is carried off directly to the drains without appearing in the streets. This must be a considerable proportion of the whole, since the streets themselves, allowing them to be ten yards wide on an average, would seem to occupy only about one-tenth part of the entire metropolitan area, so that the rain falling directly upon the public thoroughfares will be but a tithe of the aggregate quantity. But the surface-water of the streets is increased largely by tributary shoots from courts and drainless houses, and hence we may fairly assume the natural supply to be doubled by such means. At this rate the volume of rain-water annually poured into and upon the metropolitan thoroughfares by natural means, will be between five and six thousand millions of gallons, or one hundred times the quantity that is daily supplied to the houses of the metropolis by mechanical agency.

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.

“The following table exhibits the quantity of matter (both in solution and in solid state) contained in an imperial gallon of each specimen.

“STREET WATERS.

Number of Bottle.	Name of Street.	Quality of Paving.	Quality of Traffic.	Residue in an Imperial Gallon.
Number of Bottle.	Name of Street.	Quality of Paving.	Quality of Traffic.	Soluble.	Insoluble.	Both.
				Grains.	Grains.	Grains.
1	Duke-street, Manchester-square	Macadam	Middling	92·80	105·95	198·75
7	Foley-street (upper part)	„	Little	95·13	116·30	211·43
5	Gower-street	Granite	Middling	126·00	168·30	294·30
12	Norton-street	„	Little	123·87	3·00	126·87
3	Hampstead-road (above the canal)	Ballasted	Great	96·00	84·00	180·00
4	Ferdinand-street	„	Middling	44·00	48·30	92·30
2	Ferdinand-place	„	Little	50·80	34·30	85·10
10	Oxford-street	Granite	Great	276·23	537·10	813·33
6	„	Macadam	„	194·62	390·30	584·92
11	„	Wood	„	34·00	5·00	39·00

“The influence of the quality of the paving on the composition of the drainage water,” says Professor Way, “is well seen in the specimens Nos. 10, 6, and 11, all of them from Oxford-street, the traffic being described as ‘Great.’

“The quantity of soluble salts is here found to be greatest from the granite matter from the macadamized road, and very inconsiderable from the wood pavement.

“The same relation between the granite and macadam pavement seems to hold good in the other instances; the granite for any quality of traffic affording more soluble salts to the water than the macadam.

“The ballasted pavement holds a position intermediate between the macadam and the wood, giving more soluble salts than the wood, but less than the macadam.

“The quantity of solid (insoluble) matter in the different samples of water, which is a measure of the mechanical waste of the different kinds of pavement, appears also to follow the same relation as that of the soluble salts; that is to say, granite greatest, next macadam, then ballasted, and, lastly, wood pavement, which affords a quantity of solid deposit almost too small to deserve notice.

“The influence of the quality of traffic on the composition of the different specimens of drainage is well marked in nearly all cases; the greatest amount of matter both insoluble and soluble being found in the water obtained from the streets of great traffic.

“The following table shows the composition of the soluble salts of four specimens, two of them being from the granite, and two from the macadam pavement.

“It appears from the table that the granite furnishes little or no magnesia to the water, whilst the quantity from the macadam is considerable.

“On the other hand, the quantity of potash is far greatest in the water derived from the granite.

“The traffic, as was before seen, has a very great influence on the quantity of the soluble salts. It seems also to influence their composition, for we find no carbonates either in the water from the granite, or that from the macadam, where the traffic is little; whereas, when it is great, carbonates of lime and potash are found in the water in large quantity, a circumstance which is no doubt attributable to the action of decaying organic matter on the mineral substances of the pavement.

“ANALYSIS OF THE SOLUBLE MATTER IN DIFFERENT SPECIMENS OF STREET DRAINAGE WATER.

	Grains in an Imperial Gallon.
	Great Traffic.		Little Traffic.
	Granite. No. 10.	Macadam. No. 6.	Granite. No.12.	Macadam. No. 7.
Water of combination and some soluble organic matter	77·56	29·07	22·72	13·73
Silica	·51	2·81	...	...
Carbonic Acid	15·84	12·23	None	None
Sulphuric Acid	36·49	38·23	46·48	34·08
Lime	6·65	13·38	25·90	16·10
Magnesia	None	23·51	Trace	3·50
Oxide of Iron and Alumina, with a little Phosphate of Lime	2·58	1·25	...	...
Chloride of Potassium	None	10·99	None	2·79
„ Sodium	53·84	44·88	18·44	19·70
Potash	82·76	18·27	8·75	5·23
Soda	...	...	1·58	...
	276·23	194·62	123·87	95·13

“The insoluble matter in the waters consists of the comminuted material of the road itself, with small fragments of straw and broken dung.

“The quantity of soluble salts (especially of salts of potash) in many of these samples of water is quite as great, and in some cases greater, than that found in the samples of sewer-water that have been examined; and it is open to question and further inquiry, whether the water obtained from the street-drainage of a crowded city might not often be of nearly equal value as liquid manure with the sewer-water with which it is at present allowed to mix.”

With regard to the “ballasted pavement” mentioned by Professor Way, I may observe that it cannot be considered a street-pavement, unless exceptionally. It is formed principally of Thames ballast mixed with gravel, and is used in the construction of what are usually private or pleasure walks, such as the “gravel walks” in the inclosures of some of the parks, and upon Primrose-hill, &c.