RAIN

(Ger. regen.) In Meteorology, water falling from the atmosphere in drops. The theory of rain is not yet very satisfactorily established. One of the most ingenious explanations, and indeed the only one which rests entirely on known principles, was proposed by Dr. James Hulton in the Philosophical Transactions of Edinburgh, for 1787. It is now a well ascertained fact, though it was only matter of conjecture to Dr. Hutton, that the capacity of air for moisture, or the quantity of moisture which a given volume of air will hold, increases with the temperature, but in a much faster ratio than the temperature; and hence it follows that if two equal portions of air at different temperatures completely saturated with moisture are mingled together, a precipitation must take place in consequence of the mixture, which will have the mean temperature of the two portions, being unable to sustain the mean quantity of vapour. For example, suppose that while the temperature increases in an arithmetical ratio, the capacity for retaining moisture increases in a geometrical ratio, and that at the temperature of 15 centesimal degrees air can hold 200 parts of moisture, then at 30 degrees it will hold 400 parts, and at 45 degrees 800 parts. Now, suppose two equal bulks of damp air, at the respective temperatures of 15 and 45 degrees, to b« mixed together, the compound must contain 1000 parts of vapour, or either half of it 500 parts; and the temperature of the compound will bo 30 degrees. But at this temperature the air is saturated with 400 parts, and consequently there will be a precipitation of 100 parts from each of the given bulks. It is obviously not nece ary that the commingled por [s. 1040]