STATE HORTICULTURAL SOCIETY. 1 69 



especially with trees, or barren naked plains? Even dumb animals know, 

 for they fly from the scorching rays of the sun in the open fields, to the 

 grateful cooling shades of the forest. While, therefore, the great physical 

 law remains in force, that rain can only fall from a cooling atmosphere, 

 and as long as it is a physical foct that the shade is cooler than the sun- 

 shine, so long must the amount of rainfall on timbered countries be 

 greater than that on naked plains. 



Precipitation depends on saturation, and saturation upon temperature. 

 Generally, while the temperature of the air is rising, saturation can not 

 take place; but when it is falling, the point of saturation is reached at 

 some time, and rain begins to fall and continues to fall as long as the 

 temperature descends. Careful experiments show that air at the temper- 

 ature of 33 degrees Fahrenheit, when saturated, holds 1-160 part of its 

 volume of aqueous vapor in suspension. At 59 degrees, it is saturated 

 by i-8o part of its volume; and at 86 degrees, by 1-40 part of its volume 

 of vapor. Hence, every time the temperature of the air is raised 27 

 degrees Fahrenheit, its capacity to absorb and hold vapor in suspension 

 is doubled. Therefore, air saturated at S6 degrees holds in suspension 

 four times the quantity of water as when saturated at 32 degrees. 



We have seen that forests are cooler during the heats of summer than 

 open fields or naked plains; consequently there must be a difference in 

 the temperature of their respective superincumbent atmospheres. 

 If the vapor be equally distributed in their respective atmospheres, 

 then that over the forest is nearer the point of saturation than 

 that over the adjacent plains ; and when that over the forest is 

 saturated, that over the adjacent plain is not so by the ratio of the 

 difference between their respective temperatures. But during the 

 day the sun's rays increase this difference, movements of air set in, and 

 the genc*'al tempei"ature of the whole atmosphere is raised. Thus still 

 greater quai^tities of vapor are taken up and held in suspension. This 

 process may go on for days and weeks, and even months, until saturation 

 at a high temperature is attained — then a reaction sets in, perhaps by the 

 intervention of a colder stratum of air, or by drifting over a cooler surface, 

 or which is more probable, by some change in the higher regions of the 

 atmosphere b)'^ ^^•hich the transmutation of heat into electricity is 

 facilitated. Whatever may be the approximate cause, the reaction is 

 violent in proportion as the phenomenon was abnormal by which satura- 

 tion was attained. Tornadoes, hailstorms, waterspouts, and deluging 

 rains are the natural and inevitable consequences of such abnormal 

 conditions. As might be expected, these hydrometeors mainly appear, 

 and develop their most destructive forms in the vicinity where large areas 

 of land have been denuded of vegetation and exposed to the direct 

 influence of the sun. The pleasant, cool, bracing, and often chilling 

 vveatlier, that follow these violent commotions, show that they were 

 brought about by some sudden cooling process in the atmosphere. 



As the main pliysical law of meteorology concerned in the production 

 of rain mu^t now be sufficiently clear to comprehend and explain the 

 facts produced by its operation in various parts of the world within the 

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