48 THE TREES OF AMERICA. 



course, it lies level, there, the ground beneath is frozen only to a slight depth. 

 When a thaw commences, the porous nature of the soU, and the little hollows, 

 which are between the growing trees and beneath the fallen ones, retain the 

 water until the earth is thawed ; when it slowly trickles through its pores to the 

 small streams and ponds which supply the rivers, which by this process are kept 

 in an equal state of flow. But in an open country, the surface of the ground 

 is more regular and the soU more compact, while in winter it is frozen to a 

 much greater depth ; and as the snow is usually blown into heaps, the frost pen- 

 etrates irregularly, so that in the bare spots a long time is required before it can 

 be thawed. Under these circumstances, when the weather becomes warm or 

 rain falls, the water must at once concentrate in the ponds and small streams. 

 These, in their turn, pour out their overflowing contents, and thus cause the 

 terrible freshets which every year devastate portions of our country. In addi- 

 tion to all this, the unobstructed blowing of a warm wind wiU dissolve the snow 

 more than ten times as fast as when, at the same temperature, it is protected from 

 the wind. To determine this point, duringthe winter of 1856-7, 1 tried, among 

 others, the following experiments : — 



First, a body of snow, one foot in depth and sixteen feet square, was protected 

 from the wind by a tight board fence about five feet high, while another body 

 of snow, much more sheltered fr'om the sun than the first, six feet in depth and 

 about sixteen feet square, was fully exposed to the wind. When the thaw came 

 on, which lasted about a fortnight, the larger body of snow was entirely dissolved 

 in less than a week, while the smaller body was not wholly gone at the end of 

 the second week. 



Experiment second. Equal quantities of snow were placed in vessels of the 

 same kind and capacity, the temperature of the air being seventy degrees. In 

 the one case, a constant current of air was kept passing over the open vessel, 

 while the other was protected by a cover. The snow in the first was dissolved 

 in sixteen minutes, while the latter had a small unthawed portion remaining at 

 the end of eighty-five minutes ; that is, it melted five and one third times as fast 

 in the first case, where a feeble current of air was kept up, as in the other, in 

 which the snow was covered from the air. 



