AGEICULTUEAL DEAINAGE. 429 



saturated.* TJnder-drains, tlien, 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. 



Effects of Draining Lake of EaaHem. 



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 without 

 direct proof in support of theoretical and a priori conclusions. 

 Thermometrical observations have been regularly made at Zwan- 

 enburg, 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 fin- 

 ished, and the following summer had completely dried the newly 

 exposed soil — and of course greatly diminished the water-sur- 

 face — a change took place in the relative 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 

 lower than at the Helder. From the end of 1852 the thermome- 

 ter at Zwanenburg has stood, from the 11th of April to the 20th 

 of September, twenty-two hundredths of a degree higher than 

 that at Helder; but from the 14th of October to the lYth of 

 March, it has marked one-tenth of a degree lower than its mean 

 between the same dates before 1853.f 



* 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 in a year. He cites observations by Parkes, 

 showing a difference in temperature of S-o*^ [centigi'ade ?] in favor of drained, 

 as compared with undrained, ground in the same vicinity. — Instructions 

 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 effective hygroscopicity of the soil 

 increases its absorbent action, and the condensation of atmospheric vapor thus 

 produced is attended with the manifestation of heat. 



f Krecke, Het Klimaat van Nederland, ii. , p. 64. 



