449 ACTION OF MARSHES. 
in but a very small degree the compensating meteorological 
action which I have ascribed to large expansions of deeper water. 
The direct rays of the sun and the warmth of the atmosphere 
penetrate to the soil beneath, and raise the temperature of the 
water which covers it; and there is usually a much greater 
evaporation from marshes than from lakes in the same region, 
during the warmer half of the year. This evaporation implies 
refrigeration, and consequently the diminution of evaporation 
by the drainage of swamps tends to prevent the lowering 
of the atmospheric temperature, and to lessen the frequency 
and severity of frosts. Accordingly it is a fact of experience 
that, other things being equal, dry soils, and the air in contact 
with them, are perceptibly warmer during the season of vegeta- 
tion, when evaporation is most rapid, than moist lands and the 
atmospheric stratum resting upon them. ‘Instrumental obser- 
vation on this special point has not yet been undertaken on a 
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 
New England often escape frosts when the Indian-corn harvest 
on moister grounds, five hundred or even a thousand feet lower, 
is destroyed 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 example, 
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, if not carried off by the drains, might be 
* “« The simplest backwoodsman knows by experience that all cultivation is 
impossible in the neighborhood of bogs and marshes. Why is a crop near the 
borders of a marsh cut off by frost, while a field upon a hillock, a few stone’s 
throws from it, is spared?”—Lars Ley Lasrapius, Om Uppodlingar t 
Lappmarken, pp. 69, 74. 
