78 EFFECTS OF F0BREST3 ON MABSHE8. 



This is far from astonisbing when one thinks of the enormous volume 

 of water transformed by forests into vapour, and the quantity of heat 

 absorbed in this transformation. This heat must have been obtained 

 somewhere — perhaps from the soil of the forest and that of the 

 neighbourhood. 



** In the same way, there should be great damp in the neighbour- 

 hood of forests, especially when the temperature is high, and it 

 cannot be otherwise on account of the enormous amount of water in 

 the form of vapour which is discharged by forests into the adjacent 

 atmosphere. 



" This vapour is emitted in much greater abundance during the 

 day than during the night. Towards night, a little after sunset, 

 when the general temperature begins to fall, the transpiration not 

 yet having had time to slacken, and ascending into a colder air, 

 changes into visible fog, like our own breath in like circumstances 

 and this fog in its turn becomes a cloud on the following morning, 

 when the sun warms its particles ; but whether clouds or fogs they 

 will be cai-ried away by the first breeze, to descend in showers far 

 from the place of their birth. 



" It is evident that the rain of these showers, pumped up, in the 

 first place, by the tree-roots from the forest soil, if it be sometimes 

 restored to this soil it is only in part ; so that even in this respect we 

 can assert that, with regard to water, forests place the soil in a much 

 worse position than a crop of cereals. 



" If these details as to the formation of forest-fogs be correct, such 

 fogs should be more frequent in calm weather, when the air is 

 naturally more moist, and especially when the contrast is greatest 

 between the cool of the evening and the heat of the day. The test 

 of conditions for the formation of thick forest-fogs is especially com- 

 plete, at least in our climate, towards the end of summer and the 

 first half of autumn ; and it is during this period that the phenomenon 

 is most frequent and noticeable. 



" If the transpiration carried on by the leaves were coloured and 

 perceptible, it would be a grand sight to see great columns of vapour 

 ascending majestically into the air, diminishing by their height the 

 distance between the tops of the trees and the stormy clouds ; and as 

 this vapour facilitates the passage of electricity, by inci'easing the 

 moisture of the air with which it mingles, the frequency with which 

 isolated trees are struck with lightning can be accounted for. Per- 

 haps, also, if attention were turned to this point, the reason would be 

 discovered why so many people, taking shelter under trees, are struck 



