484 THE POPULAR SCIENCE MONTHLY. 



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 nu- 

 merous, 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 quan- 

 tity of water, and a cloud may be formed. If they are few, or if 

 the total 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 tem- 

 perature 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 at- 

 mosphere ? 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 pro- 

 posed 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 



