364 EFFECTS OF DRAINING ON TEMPERATURE. 
dry soils, and the air in contact with them, are perceptibly 
warmer during the season of vegetation, when evaporation is 
most rapid, than moist lands and the atmospheric stratum 
resting upon them. Instrumental observation on this special 
point has not yet been undertaken on a very large scale, but 
still we have thermometric data sufficient to warrant the general 
conclusion, and the influence of drainage in diminishing the 
frequency of frost appears to be even better established than a 
direct increase of atmospheric temperature. The steep and 
dry uplands of the Green Mountain range in Hew England 
often escape frosts when the Indian corn harvest on moister 
grounds, five hundred or even a thousand feet lower, is de¬ 
stroyed or greatly injured by them. The neighborhood of a 
marsh is sure to be exposed to late spring and early autumnal 
frosts, but they cease to be feared after it is drained, and this 
is particularly observable in very cold climates, as, for ex¬ 
ample, in Lapland.* 
In England, under-drains are not generally laid below the 
reach of daily variations of temperature, or below a point from 
which moisture might be brought to the surface by capillary 
attraction and evaporated by the heat of the sun. They, there¬ 
fore, like surface drains, withdraw' from local solar action much 
moisture which would otherwise be vaporized by it, and, at 
the same time, by drying the soil above them, they increase its 
effective hygroscopicity, and it consequently absorbs from the 
atmosphere a greater quantity of water than it did when, for 
want of under-drainage, the subsoil was always humid, if not 
saturated. Under-drains, then, contribute to the dryness as 
well as to the warmth of the atmosphere, and, as dry ground 
is more readily heated by the rays of the sun than v'et, they 
tend also to raise the mean, and especially the summer tem¬ 
perature of the soil. 
* “ The simplest backwoodsman knows by experience that all culti¬ 
vation is impossible in the neighborhood of bogs and marshes. "Why is a 
crop near the borders of a marsh cnt off by frost, while a field npon a 
hillock, a few stone’s throws from it, is spared ? ”— Laes Levi L^estadiub, 
Om Uppodlingar i Lappmarken , pp. 69, 74. 
