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the one side, and diminishes that on the other; and the sides of a valley, in like manner, receive more or less than the quantity due to a level district.

4. The natural supply is, moreover, modified in effect by the structure of the surface on which it falls. Thus, upon a rock-surface, (such as that presented by mountains,) which resists percolation, the rain collects in masses, floods itself through a fissure, or wears a channel along the line of the most pervious formation, and reaches the lower plains in the formidable rush of a mountain torrent. And as, generally, the effect of the natural supply of water is in proportion to the comparative impermeability of the soil, it follows, that the value of this supply in any district is further conditional on the structural character of the adjacent districts. Thus, from a higher impermeable district it will receive, and to a lower more permeable district it will give. Natural supply is hence, in effect, determined by the geographical situation, the superficial character, and the geological structure of a district, modified also by the structure of the surrounding district.

5. The quantity of rain that falls annually at several places, has been observed, and recorded as follows:

In England, the mean annual depth of the eight years 1836 to 1843, both included, was 26.61 inches, having varied between the extremes of 21.1 and 32·1 inches. The average annual fall at some other places has been recorded as follows:

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Humboldt has assigned the fall of rain to vary with the

latitude, being greatest at the equator, and diminishing

towards the poles in the following ratio: viz., 96 inches annually in the equatorial zone; 80 inches to latitude 20°; 29 inches to latitude 45°, and 17 inches to latitude 60o.

6. The quantity of rain thus varying, with some reference to the latitude, also to the position of the district in relation to the sea, and varying also from one year to another, is further affected by the season. Thus, the mean fall per month on an average of eight years in some districts of England has been recorded as fluctuating from 1.617 inches in March to 3.837 inches in November; the fall in each month being as follows:

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7. This monthly quantity, being the mean of eight years, does not by any means indicate the monthly proportion for any one year, the variation being as great between the same months of different years, as it is from one year to another, or indeed from one latitude to another. Thus, during the eight years over which these observations extended, the quantity of rain falling in each month was as follows:

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May June July August

September

0.70 0.94 0.84

1.22 2.62

1.68 1.85 5.00

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1.80 1.86 2.85 3.31
2-29 1.30 2.35 4.36
2-24 3.00 0.95 3.65 1.90
2.60 1.38 2:47 3.22 2:31 4.00 4.50 0.63
4.55 1.55 2.68 1.68 1.50 4.40 1.41 4.82

1.33

3.00 2.00

1.56

1.68

2.80 1.93

2.09

3.62 1.40

2.66

October.

November

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December

2.21 1.70 1.58 3.02 0.40 2.30 1.52 0.40

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Quantity in each 31.00 21-10 23-13 31-28 21-44 32-10 26-43 26-47

year

The greatest and least quantity falling in each month during this period is thus stated :—

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The third column shows the difference between the

greatest and least fall in each month during the eight years, and thus represents the relative variableness of

each month's rain. It thus appears that the fluctuation is least in February, and greatest in May.

8. As evidence of the great difference of quantity of rain which falls in similar latitudes, we may quote the following observations referring to the upland districts about Manchester, which we have compiled from Tables given by Mr. Homersham in his "Report on the Supply of Surplus Water to Manchester," &c.* These observations were made at eight stations during the four years 1844, 1845, 1846, and 1847; and at five other stations during the year 1847 only. The first column gives the name of the station at which the observations were made; the second shows its elevation above the mean level of the sea; the next five columns contain the depth of the rain in inches and decimal parts for each year, and mean depth of the four years; and the last column gives the names of the observers :—

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Mean of each of the four years 34-41 45-89 38 65 47-61 at eight stations

Mean of the one year at thir

teen stations.

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42.49

1500 24-80 39.80 37.10 35.70 34.35 R. Mathews.

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29.50 29.50
35.85 35.85

J. Meadows.

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Ditto.

* Weale. 1848.

Among the many observations made upon this subject, it must, however, be admitted that we have not yet the means of instituting any very satisfactory comparison. To do this we require careful observations carried on for a long series of years, at stations selected for the purpose, and with apparatus of the same construction.

9. Any attempt to describe the several fluctuations which are observed in the quantity of rain falling, or to explain the causes of these fluctuations, beyond the few leading circumstances we have noticed, would involve an elementary inquiry into the phenomena of rain far exceeding the limits of these pages. But we may quote a few words from the celebrated Dalton, which will be fully suggestive to the studious mind in this interesting department of meteorological science. "The cause of rain, therefore, is now, I consider, no longer an object of doubt. If two masses of air of unequal temperatures, by the ordinary currents of the winds, are intermixed, when saturated with vapour, a precipitation ensues. If the masses are under saturation, then less precipitation takes place, or none at all, according to the degree. Also, the warmer the air, the greater is the quantity of vapour precipitated in like circumstances." "Hence the reason why rains are heavier in summer than in winter, and in warm countries than in cold."*

10. The depth of rain which falls is ascertained by receiving it in a vessel of some form with a gauge connected, in which the depth may be accurately measured; but no instrument of the kind yet devised can be considered as entirely satisfactory in its action,

* Memoirs of the Literary and Philosophical Society of Manchester; vol. iii., second series, 1819, p. 507. Several valuable papers, with detailed observations made during long series of years, by Dr. Dalton and others, are to be found in these Memoirs.

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