ON RAINMAKING. 
By Alexander Macfarlane, D.Sc., LL.D., Professor of Physics in 
The University of Texas. 
Bead December 31, 1892. 
In this paper I propose first to state briefly what is known about the 
formation of rain, and then to discuss in the light of that knowledge the 
different methods of rainmaking which hqve recently been tried or pro¬ 
posed. While the text books and memoirs of physical science contain a 
great amount of sound knowledge on the subject, we nevertheless see and 
hear of professional rainmakers who are no better than the medicine 
man of the Indians, and we also witness government appropriations ex¬ 
pended in operations very suitable to the fourth of July, but useless as 
means of extending our knowledge of the formation of rain in the atmo¬ 
sphere. 
If a dish filled with water be placed inside a glass receiver, vapor will 
rise from the water until there is a certain amount of vapor in each cubic 
inch of the air; the evaporation then stops. The amount of vapor per 
cubic inch which is sufficient to saturate depends on the temperature of 
the enclosure; the higher the temperature, the greater is the amount of 
vapor required; not only so, the amount required increases more rapidly 
than the temperature. But the amount required to saturate is independ¬ 
ent of the density of the enclosed air; when the air is rare, saturation 
takes place quicker, but there is finally just the same amount of vapor 
in each cubic inch. This is true even when the enclosure is free of air. 
For every temperature of the air there is a certain maximum amount of 
aqueous vapor which it can hold per cubic inch; and conversely, for a 
given amount of aqueous vapor per cubic inch there is a certain temper¬ 
ature at which the air can just hold it. If the temperature is further 
lowered, some of the vapor condenses, and the condensed vapor may ap¬ 
pear as fog, cloud, mist, or rain. 
In what ways may a portion of the air of the atmosphere be cooled ? 
It may come into contact with another portion at a lower temperature, 
and lose heat by convection and conduction; or it may radiate some of 
its heat into space or to colder bodies; or it may use up some of its heat 
in expanding to a larger volume. The last, called dynamic cooling, 
may be observed in the working of an ordinary air pump. At the be¬ 
ginning the glass receiver is full of air having the temperature and 
moisture of the air of the room. After a few strokes made in rapid suc¬ 
cession a cloud forms inside, and drops of water trickle down the inside 
surface of the glass. The air left after a stroke expands to fill up the 
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