33 



destroy miasmata by a chemical action, probably oxidation by the oxygen and the ozone 

 which they generate. This effect is well marked in the case of Eucalypts, which have 

 rendered salubrious certain fever-stricken districts of Italy ; but perhaps Eucalypts owe 

 their virtue to the volatile antiseptic oil which they contain. Dr. Cleghorn gives it 

 that " no point has been more clearly established than the salubrious and fertilising 

 effect of forest clothing on the climate of India. It has been the subject of much 

 inquiry, and has been affirmed and demonstrated in reports from many districts.'' 

 According to Dr. H. Rogers, the cutting down of forests has been found to affect 

 seriously the healthiness of Mauritius. 



CLIMATIC INFLUENCE OF FORESTS. It is a matter of common observation that 

 forests are more damp than the open land. Ebermayer finds that the excess in the 

 humidity of the air of Bavarian forests amounts to 5' 7, 9'3, 5'2, and 5'2 per cent, 

 respectively in the four seasons of the year beginning with spring. Over a forest it 

 averages 13 per cent, according to Fautrat, and Tissandier's balloon observations show 

 that the difference is still very marked 2,500 feet above. The shaded soil of a forest 

 being less heated * and better protected by a cool and still stratum of air, **also remains 

 more moist than the open land. The loss from direct evaporation in a forest averages 

 only one-third (one-half in winter and one-fifth in summer) of that in land outside ; 

 and if the forest soil be covered with moss and decayed leaves, the proportion is even 

 doubled. 



If forests lose little moisture by direct evaporation from the soil, they exhale through 

 foliage an immense quantity of vapour into the atmosphere. Schleiden and other 

 observers have come to the startling conclusion that a wood exhales more moisture than 

 the rainfall could have supplied. fThe quantity thus evaporated varies with the difference 

 in temperature between the soil and the atmosphere, the humidity of the soil and the leaf 

 surface.ft It is less with evergreen trees than with deciduous, though the latter lose their 

 activity in winter. 



But unless water were generated by the trees themselves, infiltration, trickling, and 

 final loss by evaporation must in their sum balance the rainfall. Possibly, the evapora- 

 tion from trees has been over-estimated ; for Cailletet has shown that a plant may absorb 

 moisture directly from the atmosphere when the supply in the soil becomes inadequate 



*Bavarian observations give a difference of 7 5 Fahr. in summer, which becomes hardly appreciable in 

 winter, the mean difference for the year being 3 a 4 Fahr. 



**The diffusion of moisture in perfectly still air is extremely slow, and wind is usually the most potent 

 factor in determining evaporation. 



f From observations by Hartig and Burger, it results that oak coppice may exhale, in a year, vapour 

 corresponding to 57 inches of rainfall ; with beech, even more. 



f f Klauprecht estimates that, weight for weight, leaves of Scotch Fir evaporate 3 - 5 times less moisture 

 than those of the common oak, but Risler arrives at a smaller difference. According to Lawes and Gilbert, 

 the evaporation throughout the year, in England, is distributed thus : 



Evergreen Trees. Deciduous Trees. 



January to April ... ... 23 percent. ... ... 8 per cent. 



May to August ... ... 52'5 ,; ... ... T>6 



September to December ... 24'5 ... ... 36 



