“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.