92, EFFECTS OF FORESTS ON MARSHES. 



problem, aai the results of observation on the sabjeot are con- 

 flicting. Schuebler found that at Geneva, the evaporation from 

 bare loose earth in the months 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 evaporation from water 

 was from about one and a-half to six times as great as from the 

 earth. Taking the whole year together the evaporation from the 

 two surfaces was 199j-\ liues from earth, and 536^3 lines from 

 water. Experiments by Van der Steer, at the Helder, in the years 

 1861 and 1862, showed for the former year, an evaporation of 602'9 

 millimetres from water, 1399'6 millimfetres from grounds covered 

 with clovers and other grasses ; in 1862 the evaporation from water 

 was 584'5 millimetres; from grass-ground 875'5." It is these latter 

 observations with which we have to do, and these indicate that from 

 the bog evaporation would proceed more rapidly than it would from a 

 lake of the same extent similarly situated. 



Such seem, then, the effects of forests on swamps and of swamps on 

 forests. The moisture withdrawn from the ground and passed into 

 the atmosphere by the trees, if in excess of what is supplied from 

 a higher level, and of what also falls on the spot, as rain, snow, or 

 dew, must render them dry, and to the same extent produce the 

 meteorological effect of increasing the humidity of the air ; but at 

 the same time, by shade, and by shelter, and by covering the ground 

 with fallen trees, and broken twigs, and by the humus produced from 

 the decay of these, the forests prevent extreme desiccation. And the 

 occasional appearance of marshes on the destruction of forests, even 

 when attributable to the stoppage of an outlet, may indicate what they 

 have thus accomplished. 



