Page:Popular Science Monthly Volume 45.djvu/502

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water drops falling upon a given area until all the dust particles were carried down. He found the number of dust particles to vary from 34,000 per cubic inch in pure air taken from the top of Ben Nevis to 88,346,000 per cubic inch in air taken from a room near the ceiling, and nearly 500,000,000 per cubic inch in the flame of a Bunsen burner.

The number of these dust particles in the air determines the character of the precipitation. If the dust particles are very numerous, each one becomes a nucleus for the condensation of water vapor, but only a small quantity of water will be condensed upon each one; hence the formation of the fine drops which constitute fog. If the number is smaller, as it is likely to be at a greater distance above the earth, each nucleus may receive a larger quantity of water, and a cloud may be formed. If they are few, or if the amount of condensation is great, the drops which are formed become heavy enough to fall to the ground, and rain is produced. If the nuclei are very few, rain may fall from an almost cloudless sky.

It is well known that as we ascend above the earth the temperature falls about one degree Fahrenheit for three hundred feet; consequently, while the air at the surface of the earth may be far above the dew point, the air at a few thousand feet above the earth may be cooled below the dew point. The height of the clouds always indicates the distance above the earth at which the air is cold enough for condensation to begin. The clouds, being made up of these little dust particles surrounded by water, are heavier than the air, and are slowly settling toward the earth, but as fast as the little drops settle into the warmer air, the rate of evaporation from their surface is increased, and before they have settled far the water has been evaporated off. Hence, at a given time, over an area of uniform temperature, the lower surfaces of the clouds are all at nearly the same distance above the earth.

How, then, shall rain be produced in the great unbounded atmosphere? There are but two ways. Either the total quantity of vapor in the atmosphere must be increased, or the temperature of the air must be diminished. It is probably safe to assume that there are, under all ordinary circumstances, a sufficient number of dust particles in the air to form the nuclei for condensation, so that no artificial provision need be made for these.

So far as I am aware, no enterprising rain-maker has yet proposed a method of increasing the total moisture of the air to any appreciable extent, though some of them have attempted this on the small scale, probably in the vain hope that if they touched the button Nature would do the rest. This, by the way, has been the one claim upon which all these pretenders have based their arguments. They have steadfastly and with unanimity asserted