42 EFFECTS OP FORESTS ON HUMIDITY. 



Cape Town it must pass through an intermediate stratum of air of a 

 higher temperature than that to which it has been reduced. By this 

 it has its temperature again raised, and the cloud evaporates and 

 disappears, followed by continuous masses of cloud, which on reaching 

 the same level vanish into thin air, leaving no trace behind. 



The quantity of moisture passing into the atmosphere from the 

 leaves of a forest in active vegetation must be considerable. Cal- 

 culate the number of stomata, or stomates, on a leaf, multiply this 

 by the number of leaves on a branch, the product by the number of 

 such branches on a tree, and the product of this by the total number 

 of trees in the clump, or the total number of trees in the forest, and 

 the final product will indicate the provision made for evaporation 

 from the forest. There are similar stomates on every verdant plant 

 on the dry land ; but the evaporating surface supplied by the leaves, 

 rising tier above tier, far exceeds in extent that supplied by the 

 herbage and the grass growing elsewhere ; and in many places these 

 may be found growing as luxuriantly on the soil of the forest as in 

 the fields beyond, or perhaps more so, and adding their quota of 

 evaporation to the evaporation from the trees. 



Of the moisture thus raised by the tree, and no longer required 

 when the sap has been elaborated in the leaf, the air will only take 

 up what quantity it can, at the temperature at that time and place, 

 dissolve and hold in solution ; and cases have been cited in which the 

 excess is so great that the leaves seem to act as alembics, distilling 

 water which falls in great drops to the ground. 



Where this does not take place, what the air dissolves it will hold 

 in solution so long as the temperature is maintained at the same or a 

 higher point ; but if the temperature fall below the point at which it 

 can do this, what it cannot sustain as invisible vapour will be 

 deposited or suspended in the form of mist, or cloud ; and such a 

 reduction, may follow the setting of the sun, or even the decline of it 

 in the afternoon and towards nightfall ; or if there come over the 

 trees a wind in any degree colder than the air in which they are 

 enveloped, the air is thereby cooled down, and a quantity of the 

 moisture which it held in solution may be deposited in the form of 

 fog, or of dew, or of rain. 



But this is not all ; we shall afterwards have occasion to consider 

 more fully the effect of vegetation on temperature, but here it may 

 be remarked that it will generally be found that the temperature, 

 both of the earth and of the atmosphere, is lower amidst abundant 



