METEOROLOGICAL EFFECTS OF DRAINING. 427 



liand, if the volume of water abstracted is great, its removal de- 

 prives its basin of an equalizing and moderating influence ; for 

 large bodies of water take very slowly the temperatui'e of the air 

 in contact with their surface, and are almost constantly either 

 sending off heat into the atmosphere or absorbing heat from it. 

 Besides, as we have seen, lakes in elevated positions discharge 

 more or less water by infiltration, and contribute it by the same 

 process to other lakes, to springs, and to rivulets, at lower levels. 

 Hence the draining of lakes, on a considerable scale, must modify 

 both the humidity and the temperature of the atmosphere of the 

 neighboring regions, and the permanent supply of ground-water 

 ior the lands lying below them. 



Meteorological Action of Marshes. 



The shallow water of marshes, indeed, performs this latter 

 function, though, under ordinary circumstances, marshes exercise 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 pene- 

 trate 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 



of December, January and February, was from two and a half to nearly six 

 times as great as from a like surface of water. In the other months, the evap- 

 oration from water was from about one and a half to six times as great as 

 from earth. Taking the whole year together, the evaporation from the two 

 surfaces was 199^*^ lines from earth and SSe^ig lines from water. Experiments 

 by Van der Steer, at the Helder, in the years 1861 and 1863, showed, for the 

 former year, an evaporation of 603.9 millimetres from water, 1399.6 millimetres 

 from ground covered with clover and other grasses ; in 1863, the evaporation 

 from water was 584.5 millimetres, from grass-ground, 875.5. — Wilhelm, Der 

 Boden und das Wasser, p. 57 ; Krecke, Het Klimaat van Nederland, ii., p. 111. 

 On the other hand, the evaporation from the Nile in Egypt and Nubia is 

 stated to be three times as great as that from an equal surface of the soil which 

 borders it. — LoMBARorNi, Saggio Idrologico sul Nilo, Milano, 1864, and Ap- 

 pendix. The relative thermometrical conditions of land, air and water in the 

 same vicinity are constantly varying, and the hygrometrical state of the two 

 former is equally unstable. Consequently there is no general formula to 

 ■express the proportionate evaporation from fluid and solid geographical sur- 

 faces. 



