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.