444 POPULAR SCIENCE MONTHLY. 



on grasses, estimated that an acre of grass emits into the atmosphere 

 6.400 quarts of water in twenty-four hours. 



It is evident, therefore, that vegetation tends powerfully to cool 

 the atmosphere during a summer day, and this effect increases in pro- 

 portion to the increase of the temperature. The influence of trees 

 heavily leaved, in a district where there is no other vegetation, in 

 moderating and equalizing the temperature, can not be overesti- 

 mated. The amount of superficial surface exposed by the foliage of 

 a single tree is immense. For example, " the Washington elm, of 

 Cambridge, Mass., a tree of moderate size, was estimated several 

 years since to produce a crop of seven million leaves, exposing a sur- 

 face of two hundred thousand square feet, or about five acres of 

 foliage." 



Trees regulate the humidity of the air by the process of absorp- 

 tion and transpiration. They absorb the moisture contained in the 

 air, and again return to the air, in the form of vapor, the water which 

 they have absorbed from the earth and the air. The flow of sap in 

 trees for the most part ceases at night, the stimulus of light and heat 

 being necessary to the function of absorption and evaporation. Dur- 

 ing the heated portions of the day, therefore, when there is the most 

 need of agencies to equalize both temperature and humidity, trees 

 perform their peculiar functions most actively. Moisture is rapidly 

 absorbed from the air by the leaves, and from the earth by the roots, 

 and is again all returned to the air and earth by transpiration or 

 exudation. The effect of this process upon temperature and humid- 

 ity is thus stated by Marsh : " The evaporation of the juices of the 

 plant by whatever process effected, takes up atmospheric heat and 

 produces refrigeration. This effect is not less real, though much less 

 sensible in the forest than in meadow and pasture land, and it can 

 not be doubted that the local temperature is considerably affected 

 by it. But the evaporation that cools the air diffuses through it, 

 at the same time, a medium which powerfully resists the escape of 

 heat from the earth by radiation. Visible vapor or clouds, it is well 

 known, prevent frosts by obstructing radiation, or rather by reflect- 

 ing back again the heat radiated by the earth, just as any mechan- 

 ical screen would do. On the other hand, clouds intercept the rays 

 of the sun also, and hinder its heat from reaching the earth." Again, 

 he says, upon the whole, their general effect " seems to be to mitigate 

 extremes of atmospheric heat and cold, moisture and drought. They 

 serve as equalizers of temperature and humidity." 



Again, let us notice the effects of trees upon malarial emana- 

 tions. The power of trees, when in leaf, to render harmless the 

 poisonous emanations from the earth has long been an established 

 fact. Man may live in close proximity to marshes from which arise 



