46 EFFECTS OF FORESTS ON HUMIDITY. 



always in action. This influence, I believe, is to be found in the 

 equalization of the temperature of the forest, and partly in the 

 balance of the humidity exhaled by the trees, and that absorbed and 

 condensed invisibly by the earth." 



I accept the statement with all confidence in the correctness of the 

 observation by Mr Marsh. The fact was previously unknown to me. 



By the faU of leaves and their decay, and in other ways, the soil 

 in which trees grow becomes rich with humus, by which means its 

 power of attracting moisture from the earth is increased. In a forest 

 the temperature is more equable than in the open country ; to both 

 of these facts Mr Marsh refers, and I may add, what is also held by 

 him, that there is heat evolved as well as heat absorbed in vegetation, 

 a point which will afterwards be discussed more fully. In virtue 

 of this and of the shelter of a forest, it may in certain conditions 

 continue for a time, more or less protracted, protected from a change 

 of temperature, producing a fog in the open country around. 



Without ampler information, or personal observation of the fact 

 with opportunity of noting the circumstances, I am not in a position 

 to say more. 



Sect. III. — On Clouds occasionally seen surmounting Mountains while 

 the Atmosphere is otherwise Clear. 



With regard to the phenomena of clouds apparently attracted by 

 mountains, Mr Marsh, in his treatise " On the Earth as Modified by 

 Human Action," writes, — " In discussing the influence of mountains 

 on precipitation, meteorologists have generally treated the popular 

 belief that mountains attract to them clouds floating within a certain 

 distance from them as an ignorant prejudice, and they ascribe the 

 appearance of clouds about high peaks solely to the condensation of 

 the humidity of the air, carried by atmospheric currents up the slopes 

 of the mountains to a colder temperature ; but if mountains do not 

 draw clouds and invisible vapour to them, they are an exception to 

 the universal law of attraction. The attraction of the small Mount 

 Shehallien was found suflBcient to deflect from the perpendicular, by 

 a measiu-able quantity, a plummet weighing but a few ounces. Why, 

 then, should not greater masses attract thence volumes of vapour 

 weighing many tons, and floating freely in the atmosphere within 

 moderate distances of the mountains 1" 



As stated thus, the popular belief assumes a form in which it can 

 be dealt with more satisfactorily than in the vaguer form in which it 



