CLOUDS SURMOUNTING FORESTS. 45 



moisture in solution : very mucli as is the case with the sand and 

 dust filling the air immediately before the falling of the rain ; what- 

 ever proportion of these may have been brought'from a distance 

 more or less remote, most of it may have been seen raised from the 

 ground on the spot as the mighty rushing wind passed on in its 

 course ; and the little lapse of time between the appearance of this 

 precursor and the precipitation of the rain was only such as was 

 required, or at least such as was occupied, in the aggregation of the 

 rain particles into the larger drops which fell, and the precipitation 

 of these by gravitation and by the blast, aided, it may be, by the 

 co-operation of electric force, the process being essentially the same 

 whether the blast have come on like an onward moving cold wave, 

 or have advanced as an advancing whirlwind which raised the air 

 through which it passed to an elevation at which, it may be in 

 consequence of sudden expansion, the temperature was too low to 

 retain all the moisture in solution. 



With the copious evapoi-ation going on fx-om the leaves of a forest 

 there is nothing, in view of such a rainfall, surprising in any change 

 of wind producing a cloud or mist above a forest, where formerly 

 the air had been perfectly transparent ; and everything known in 

 regard to such phenomena makes it probable that in general, if not 

 invariably, the cloud is produced there, and not attracted thither by 

 the forest. 



In connection with this, there is an observation, made by Mr Marsh, 

 which demands attention : — " There is one fact," says he, " which I 

 have nowhere seen noticed, but which seems to me to have one 

 important bearing on the question, Whether forests tend to maintain 

 an equilibrium between the various causes of hygroscopic action, and, 

 consequently, to keep the air within their precincts to an approxi- 

 mately constant condition, so far as this meteorological element is 

 concerned. I refer to the absence of fog or visible vapour in thick 

 woods in full leaf, even when the air of the neighbouring open ground 

 is so heavily charged with condensed vapour as completely to obscure 

 the sun. The temperature of the atmosphere of the forest is not 

 subject to so sudden variations as that of cleared ground, but, at the 

 same time, it is far from constant, and so large a supply of vapour as 

 is poured out by the foliage of the trees could not fail to be condensed 

 into fog by the same causes as in the case of the adjacent meadows, 

 which are often covered with a dense mist while the forest air 

 remains clear, were there not some potent counteracting influence 



