428 ACTioisr of maeshes. 



■warmer half of tlie 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 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 large scale, but still we have ther- 

 mometric data sufficient to warrant the general conclusion, and 

 the influence of drainage in diminishing the frequency of frost 

 appears to be even better estabhshed than a direct increase of at- 

 mospheric 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 brought to the 

 surface by capillary attraction, and evaporated by the heat of the 

 sun. They, therefore, Hke surface-di'ains, 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 



* " 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 Le ve L^stadius, Om JJppodlingar i Lapp- 

 marken, pp. 69, 74. 



In the late spring frosts, which are sometimes so destructive to the crops in 

 Northern Italy, it is generally observed that only the low grounds are affected, 

 the high grounds very seldom suffering from that cause. 



