AGRICULTURAL DRAINAGE. 443 
brought to the surface by capillary attraction, and evaporated 
by the heat of the sun. They, therefore, 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 wet, they tend also to raise the mean, and 
especially the summer, temperature of the soil. 
Liffects of Draining Lake of Haarlem. 
The meteorological influence of the draining of lakes and of 
humid soils has not, so far as I know, received much attention 
from experimental physicists ; but we are not altogether with- 
out direct proof in support of theoretical and @ priort conclu- 
sions. Thermometrical observations have been regularly made 
at Zwanenburg, near the northern extremity of the Lake of 
Haarlem, for more than a century; and since 1845 a similar 
registry has been kept at the Helder, forty or fifty miles more 
to the north. In comparing these two series of observations, it 
is found that towards the end of 1852, when the draining of 
the lake was finished, and the following summer had com- 
pletely dried the newly exposed soil—and, of course, greatly 
* Mangon thinks that the diminution of evaporation by agricultural drain- 
age corresponds, in certain circumstances, to five per cent. of the heat received 
from the sun by the same surface ina year. He cites observations by Parkes, 
showing a difference in temperature of 5.5° [centigrade ?] in favor of drained, 
as compared with undrained, ground in the same vicinity.—Jnstructions 
pratiques sur le Drainage, pp. 227, 228. 
The diminution of evaporation is not the only mode in which under-drain- 
ing affects the temperature. The increased efiective hygroscopicity of the 
soil increases its absorbent action, and the condensation of atmospheric vapor 
thus produced is attended with the manifestation of heat. 
