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:--
----------+----------------------------------------+------------+---------
| Depth of Rain in inches. | Quantity of|Number of
+--------------+------------+------------+rain falling| days on
Months. |Royal Society,| Howard, | Daniell, | in the | which
| according to |according to|according to| different | rain
| observation. |observation.|calculation.| seasons. | falls.
----------+--------------+------------+------------+------------+---------
January | 1·56 | 1·907 | 1·483 | | 14·4
February | 1·45 | 1·643 | 0·746 | Winter. | 15·8
March | 1·36 | 1·542 | 1·440 | 5·868 | 12·7
April | 1·55 | 1·719 | 1·786 | | 14·0
May | 1·67 | 2·036 | 1·853 | Spring. | 15·8
June | 1·98 | 1·964 | 1·830 | 4·813 | 11·8
July | 2·44 | 2·592 | 2·516 | | 16·1
August | 2·37 | 2·134 | 1·453 | Summer. | 16·3
September | 2·97 | 1·644 | 2·193 | 6·682 | 12·3
October | 2·46 | 2·872 | 2·073 | | 16·2
November | 2·58 | 2·637 | 2·400 | Autumn. | 15·0
December | 1·65 | 2·489 | 2·426 | 7·441 | 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 23-1/10 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 277-1/4 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 | Admeasurement
| and tons. | in gallons.
------------------+-----------------------+-----------------------
Yearly Rainfall } | 385,399,721,220 lbs., |
in the } | or | 38,539,972,122 gals.
Metropolis } | 172,053,447 tons. |
------------------+-----------------------+-----------------------
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:--