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HOW RAIN IS FORMED.*

By H. F. BLANFORD, F. R. S.

In certain villages in the Indian Central Provinces, besides the village blacksmith, the village accountant, the village watchman, and the like, there is an official termed the gàpogàri, whose duty it is to make rain. So long as the seasons are good, and the rain comes in due sea. son, his office is no doubt a pleasant and lucrative one. It is not very laborious, and it is obviously the interest of all to keep him in good humor. But if, as sometimes happens, the hot dry weather of April and May is prolonged through June and July, and week after week the ryot sees his young sprouting crops withering beneath the pitiless hot winds, public feeling is wont to be roused against the peccant rainmaker, and he is led forth and periodically beaten until he mends his ways and brings down the much-needed showers.

You will hardly expect me, and I certainly can not pretend, to impart to you the trade-secrets of the professional rain-maker. Like some other branches of occult knowledge which Madam Blavatsky assures us are indigenous to India, this art of rain-making is perhaps not to be acquired by those who have been trained in European ideas; but we can at least watch and interrogate nature, and learn something of her method of achieving the same end; and if her scale of operations is too large for our successful imitation, we shall find that not only is there much in it that may well challenge our interest, but it may enable us to some extent to exercise prevision of its results.

Stated in the most general terms, nature's process of rain-making is extremely simple. We have its analogue in the working of the common still. First, we have steam or water vapor produced by heating and evaporating the water in the boiler; then the transfer of this vapor to a cooler; and finally we have it condensed by cooling, and reconverted into water. Heat is communicated to the water to convert it into vapor, and when that heat is withdrawn from it, the vapor returns to its original liquid state. Nature performs exactly the same process. In the still, the water is heated until it boils; but this is not essential,

A lecture delivered at the Hythe School of Musketry on November 19, 1888.— (Nature, January 3, 1889, vol. XXXIX., pp. 224–229.)

for evaporation may take place at all temperatures, even from ice. A common little piece of apparatus, often to be seen in the window of the philosophical instrument maker, and known as Wollaston's cryophorus, is a still that works without any fire. It consists of a large glass tube with a bulb at each end, one of which is partly filled with water; and, all the air having been driven out of the tube by boiling the water, it is hermetically sealed and allowed to cool. It then contains nothing but water and water vapor, the greater part of which re-condenses when it cools. Now, when thus cold, if the empty bulb be surrounded by ice, or, better, a mixture of ice and salt, the water slowly distils over, and is condensed in the colder bulb, and this without any heat being applied to that which originally contained the water. And this shows us that all that is necessary to distillation is that the condenser be kept cooler than the evaporator.

Nevertheless, at whatever temperature it evaporates, water requires heat, and a large quantity of heat, merely to convert it into vapor; and this is the case with the cryophorus; for if the evaporating bulb be wrapped round with flannel, and so protected from sources of heat around, the water cools down until it freezes. That is to say, it gives up its own heat to form vapor. A simple experiment that any one may try with a common thermometer affords another illustration of the same fact. If a thermometer bulb be covered with a piece of muslin, and dipped into water that has been standing long enough to have the same temperature as the air, it gives the same reading in the water as in the air. But if when thus wetted it be lifted out and exposed to the air, it begins to sink at once, owing to the evaporation of the water from the wet surface, and it sinks the lower the faster it dries. In India, when a hot wind is blowing, the wet bulb sometimes sinks 40° below the temperature of the air.

Now this is a very important fact in connection with the formation of rain, because it is owing to the fact that water vapor has absorbed a large quantity of heat, (which is not sensible as heat, but must be taken away from it before it can be condensed and return to the liquid state,) that vapor can be transported as such by the winds for thousands of miles, to be condensed as rain at some distaut part of the earth's surface.

I have said that the quantity of absorbed heat is very large. It varies with the temperature of the water that is evaporating, and is the greater the lower that temperature. From water that is on the point of freezing it is such that 1 grain of water absorbs in evaporating as much heat as would raise nearly 5 grains from the freezing to the boiling point. This is called the latent heat of water vapor. As I have said, it is quite insensible. The vapor is no warmer than the water that produced it, and this enormous quantity of heat has been employed simply in pulling the molecules of water asunder and setting them free in the form of vapor, which is merely water in the state of gas. All