25 



The Production op Rain. 



To quote from the excellent author I have just mentioned, " The water is not 

 motionless either in the depths of the oceanic basin, in the solid ice, or in the atmosphere. 

 Thanks to the always active power of the sun, to the aerial currents, the water rises 

 vertically from the depths of the seas to its surface, becomes vaporized at all tempera- 

 tures, ascends in the shape of invisible vapour through the ocean of the air, becomes 

 condensed into clouds, travels across continents, falls again in the shape of rain, filters 

 through the surface of the soil, passes along the strata of impermeable clay, springs up as 

 a source or fountain head, descends by the streamlet into the river, and falls from the 

 river back into the sea again." 



The vapour of water, as we have seen, rises from the ocean, mingles with the dilating 

 and arising air, and in immense quantities ascends into the higher regions of the atmos- 

 phere. 



Will my readers now for a moment study this little table. It is but nine lines : — ■ 

 At 14 deg. a cubic foot of air is saturated with water by the weight of 1 grain. 

 30 

 41 

 49 

 56 

 66 

 80 

 88 

 100 



When we thoroughly comprehend the effect of the fact stated in this table, we 

 understand why two clouds or two currents of air more or less saturated with vapour of 

 water, coming into contaqt at certain temperatures, produce rain, tt occurs in the follow- 

 ing manner : — 



We will notice that a ioot of air at a temperature of one hundred (the heat of a very 

 hot day indeed) will hold twenty grains of water. If it were only at thirty degrees it 

 would hold but two grains of water. Now let us suppose a mass of a thousand cubic 

 feet of air at 100 degrees, and holding twenty thousand grains of water. Well, a cold 

 current of air comes along, meets our cubic mass, and cools it down to thirty degrees. It can 

 only hold two thousand grains now ; the cold current has served an ejectment on the odd 

 eighteen thousand grains, and they must fall out. They would fall out first into cloud, 

 then into rain, and that is a rough sketch of the way in which rain is produced. 



But we will go more slowly, and first show how a cloud is formed. Here are the 

 words of an excellent writer on the subject, so concisely put and so clearly, that we cannot 

 do better than copy them : — 



" The invisible vapour of water spread through the atmosphere becomes visible when 

 a decUne in the temperature or an addition of moisture brings it to the point of satura- 

 tion. Suppose, for instance, that a certain quantity of air at eighty-six degrees contains 

 478 grains of vapour of water, this air will be quite transparent. If by some cause or 

 other this air descends to seventy-seven degrees, or receives an accession of moisture 

 (either will do) it will become opaque. If it is done by the lowering of the temperature, 

 a diminution of nine degrees of heat will cause 108 'grains of vapour of water to be con- 

 densed and to become visible. This is what a cloud really is : vapour of water which the 



