1856.] The Absence of Trees from Prairies. 269 



deserts, whicli occupy the central portions of this continent and of Asia, 

 to their great distance from the sea, and to the position of mountain 

 ranges which intercept the supplies of aqueous vapor conveyed by the 

 winds from the great oceanic reservoir. 



It appears from metereological observations, that high mountains 

 increase considerably the fall of rain in their immediate vicinity, and 

 cause it to descend in more frequent and seasonable showers. The eifects 

 of these pinnacles of nature can not be accounted for by the mere reduction 

 of temperature which they occasion ; and it would seem that part of 

 their meteoric action arises from their discharging the upper atmosphere 

 of electricity. In extensive plains, the lower stratum of dry and dense 

 air serves as an insulator between the earth and the higher atmospheric 

 regions ; electricity, accordingly, becomes more intense at great eleva- 

 tions, and is discharged only when the air is saturated with moisture. 

 The discharge seems to be the means of condensing and precipitating 

 to the earth much of the aqueous vapor, as heavy rains and hail-storms 

 are th^ invariable associates of thunder and lightning on an extensive 

 scale. Professor Hare ascribes this effect to an indirect influence of 

 electric action. As the air parts with its superfluous electricity, it is 

 repelled from the earth ; in ascending, it is released from a considerable 

 pressure and expands ; cold is the result of this expansion ; the reduc- 

 tion of temperature which takes place causes the vapor dissolved in the 

 atmosphere to return to a liquid, or often, to a solid form, and heavy rains 

 or hail-storms are the consequence. As the lower atmospheric stratum 

 is moistened by the descending drops, the escape of the electricity is 

 facilitated, and the rain continues until the air is deprived of a vast 

 amount of vapor it previously contained, and brought into an electrical 

 condition more favorable to a state of repose. 



In an article published in the Journal of Man for February, 185.3, I 

 advanced similar views, and supposed them to be original ; but I subse- 

 quently found that the same doctrines had been broached by Dr. Hare 

 some years before. The ascent of air during heavy rains, is also main- 

 tained by Professor Espy, but is ascribed by him to the heat produced 

 by the condensing vapor. It must, indeed, be admitted that the heat 

 arising from this cause may co-operate with electricity in disturbing the 

 repose of our serial ocean ; and it seems extremely difficult to decide, 

 with any degree of precision, the part which both agencies perform in 

 meteoric phenomena. But indepedent of any theoretical deductions, all 

 the results of observation and experience disclose the very remarkable 

 peculiarity in the fall of rains, as determined by the physical features 

 of the earth's surface. In very extensive plains, rains are withheld for 



