444 DRAINING OF LAKE OF HAARLEM. 
diminished the water-surface—a change took place in the rela- 
tive temperature of those two stations. Taking the mean of 
each successive period of five days, from 1845 to 1852, both 
inclusive, the temperature of Zwanenburg was thirty-three 
hundredths of a degree centigrade /ower than at the Helder. 
From the end of 1852 the thermometer at Zwanenburg has 
stood, from the 11th of April to the 20th of September, twenty- 
two hundredths of a degree Aigher than that at Helder; but 
from the 14th of October to the 17th of March, it has marked 
one-tenth of a degree dower than its mean between the same 
dates before 1853.* 
There is no reason to doubt that these differences are due to 
the draining of the lake. In summer, solar irradiation has 
acted more powerfully on the now exposed earth and of 
course on the air in contact with it; and there is no longer a 
large expanse of water still retaining and of course impart- 
ing something of the winter temperature ; in winter, the earth 
has lost more heat by radiation than when covered by water, 
and the influence of the lake, as a reservoir of warmth ac- 
cumulated in summer and gradually given out in winter, was 
of course lost by its drainage. Doubtless the quantity of 
moisture contained in the atmosphere has been modified by the 
same cause, but it does not appear that observations have been 
made upon this point. Facts lately observed by Glaisher tend 
to prove an elevation of not far from two degrees in the mean 
temperature of England during the course of the last hundred 
years. For reasons which I have explained elsewhere, the 
early observations upon which these conclusions are founded 
do not deserve entire confidence ; but admitting the fact of the 
alleged elevation, its most probable explanation would be found 
in the more thorough draining of the soil by superficial and by 
subterranean conduits. 
So far as respects the immediate improvement of soil and 
climate, and the increased abundance of the harvests, the Eng- 
lish system of surface and subsoil drainage has fully justified 
* KRECKE, Let Klimaat van Nederland, ii., p. 64. 
