Whatever form of rain-gauge is adopted, it
must be placed in an exposed situation, at a distance from all buildings, and trees, and other objects likely to interfere with the free descent of rain into the funnel. It is usual, in rainy weather, to observe the quantity of water in the gauge every morning; but this does not seem to be often enough, considering how freely water evaporates in an exposed situation. An error may also arise from some of the water adhering to the sides of the vessel, unless an allowance is made for the quantity thus lost by a contrivance such as the following:—Let a sponge be made damp, yet so that no water can be squeezed from it, and with this collect all the water which adheres to the funnel and cylinder, after as much as possible has been drawn off; then, if the sponge be squeezed, and the water from it be received in a vessel which admits of measuring its quantity, an estimate may be made of the depth due to it; and this being added to the depth given by the instrument, would probably show correctly the required depth of rain.
When snow has fallen the rain-gauge may not give a correct quantity, as a portion of it may be blown out, or a greater quantity may have fallen
than the mouth will contain. In such cases, it is recommended to take a cylindrical tube and press it perpendicularly into the snow, and it will bring out with it a cylinder equal to the depth. This, when melted, will give the quantity of water which can be measured as before. The proportion of snow to water is about seventeen to one; and hail to water, about eight to one. These quantities, however, may vary according to the circumstances under which the snow or hail has fallen, and the time they have been upon the ground.
The rain-gauge should be placed as near the surface of the ground as possible; for it is a perplexing circumstance, that the rain-gauge indicates very different quantities of rain as falling upon the very same spot, according to the different heights at which it is placed. Thus it has been found, that the annual depth of rain at the top of Westminster Abbey was 12.1 inches nearly, while, on the top of a house sixteen feet lower, it was rather more than 18.1 inches, and on the ground, in the garden of the house, it was 22.6 inches. M. Arago has also found from observations made during twelve years, that on the terrace of the Observatory at Paris the annual depth was about
2¼ inches less than in the court thirty yards below.
It would naturally be expected from these observations, that less rain falls on high ground than at the level of the sea. Such however is not the case, except on abrupt elevations; where the elevation is made by the natural and gradual slope of the earth’s surface, the quantity of rain is greater on the mountain than in the plain. Thus, on the coast of Lancashire, there is an annual fall of 39 inches; while at Easthwaite, among the mountains in the same county, the annual depth of rain amounts to 86 inches. By comparing the registers at Geneva and the convent of the Great St. Bernard, it appears that at the former place, by a mean of thirty-two years, the annual fall of rain is about 30¾ inches; while at the latter, by a mean of twelve years, it is a little over 60 inches.
In order to explain these remarkable differences, it must not be supposed that the clouds extend down to the ground, so as to cause more rain at the foot of Westminster Abbey than on its roof. There is no doubt that in moist weather the air contains more water near the ground than a few hundred feet above it; and probably, the same cause
which determined a fall from the cloud, would also throw down the moisture floating at a low elevation. Much rain also proceeds from drifting showers, of short duration, and the current moves more slowly along the surface, and allows the drops to fall as fast as they are formed. In hilly countries, on the contrary, clouds and vapours rest on the summits without descending into the plains, and, according to some, the hills attract electricity from the clouds, and thus occasion rain to fall. Mr. Phillips supposes that each drop of rain continues to increase in size from the commencement to the end of its descent, and as it passes successively through the moist strata of the air, obtains its increase from them; while the rain which falls on the mountain may leave these moist strata untouched, so that they may, in fact, not form rain at all.
The drops of rain are of unequal size, as may be seen from the marks made by the first drops of a shower upon any smooth surface. They vary in size from perhaps the twenty-fifth to a quarter of an inch in diameter. It is supposed that in parting from the clouds they fall with increasing speed, until the increasing resistance of the air