The circumstances just mentioned correspond nearly to those which would take place about the earth if local and other accidental circum stances did not interfere with the general process. In its actual con dition, when a column of air vertically over any place is from any cause heated more than the neighbouring columns, it begins to ascend by its diminished specific gravity, the colder air of the vicinity flows in to fill up the void, and thus the relation between the temperature and humidity at the place is deranged. Then, agreeably to the general theory of Dr. Hutton, a precipitation of the vapour takes place.
In proportion to the density of the vapour, the magnitudes of the condensed particles of water are greater (this, however, does not appear to be proved, and the magnitude of the particles must be in some ratio to the temperature of condensation); in the upper regions of the air the cloud assumes a light appearance, but below it is more dense.
After their formation, the clouds are driven about by the winds, receiving new accessions of precipitated vapour till the air is no longer capable of supporting them, and then their substance descends in rain, snow, or hail.
On the supposition that the surface of the earth is without inequalities, and that the temperature gradually diminishes from the equator towards either pole, it should follow that the rarefaction of the air and the evaporation of the water, and consequently the quantity of rain, must diminish according to some law with the distances of places from the equator. Now the mean temperature in any latitude being known, the quantity of moisture in the atmospherical column at that latitude can be found, since it depends on the temperature : hence, knowing also the variations to which the temperature of the atmosphere at the place is subject in the course of the year, the mean annual depth of ram in that latitude may be computed. On such principles, Humboldt deter mined that the mean annual depth of rain should be, at the equator, 96 inches ; in lat. 45*, 29 inches ; and in lat. 60°, 17 inches. The circumstances, however, which render the temperatures in different latitudes, and even on the circumference of the Mine parallel, must produce irregularities in the quantities of rain which fall at different places; yet the results of observation show that, in proceeding from the equator towards the north polo, there is in reality a diminution In the mean annual quantities of rain.
From an average of the observations made during fourteen years (1810 to 1323 inclusive), the mean annual depth of rain on the Malabar coast is inches, and the mean annual temperature is 80.4' (Fehr.), but the annual depths of rain are very irregular, and differ considerably.
From a mean of observations for seven years (1817 to 1623 inclusive), the moan annual depth of rain at Bombay was only inches. From a mean of observations during seventeen years (1b02 to 1818 inclusive), Mr. Dalton makes the mean annual depth of rain at Manchester equal to 33'596 inches, the mean annual temperature being ; and here also the annual quantities of rain vary very irregularly. The same meteorologist estimates the average of the annual quantity of rain in England to be 311 inches ; the greatest quantity, according to the data he investigated, being at Keswick in Cumberland (— 071 inches), and the least at Upminsthr in Essex but it is supposed that this estimate of the mean quantity is higher than the truth, because too many of the observations were made in the maritime counties, where the atmosphere may be expected to be the most humid.
According to observations made by members of the British Meteorological Society, collected and aiseussed by Mr. Olaishcr, the quantity of rain that falls annually in the islands of Jersey and Guernsey is about 31 inches ; in the counties of Cornwall and Devon from 30 to 40 inches, or in some places still more; in the Isle of Wight and over the south of England about 30 inches ; about London 25 aches; about the town of Bedford and the cities of York and Durham 24 inches; near the east coast 27 inches; 30 inches near the west coast ; and about 27 or 28 inches elsewhere.
The late J. F. Miller, F.1l.S., of the Observatory, Whitehaven, Cumberland, by whom the meteorological phenomeua of the English lake district were accurately observed and recorded, (` Phil. Trans., 1851, &c.) showed that in the valleys of Cumberland and Westmore land, the usual fall of rain varies from 50 to 100 inches in depth in a year ; and these large quantities increase with the elevation, till the greatest annual depth of rain is attained at the altitude of about 1000 feet ; at higher elevations it decreases in amount. He records the very large fall of 38'9 inches in a single month, a quantity exceed ing the average annual fall of all England. The hamlet of Seathwaite, in Borrowdale, was until lately considered to be the wettest spot in Great Britain, the rain-fall, in the year 1850, amounting to 143.9G inches. But Mr. Miller ascertained that this was greatly exceeded by a new station for observation about a mile and a-half south west of Seatliwaite, and 580 feet above it, or 980 feet above the sea level, at the extreme southern termination of the valley, on the shoulder of Sprinkling Fell, or the Stye. In 1850 the rain-fall at this station amounted to 189.49 inches.