MEIiIiONI ON THE NOCTURNAL COOLING OF BODIES. 541 



minutes. This experiment is sufficient to explain the small 

 quantity of dew which is remarked under trees, in the interior 

 of hedges, and in all places where the calorific communication 

 between the sky and the earth is more or less intercepted ; the 

 radiating substances in these cases remain more or less dry, 

 because the cold resulting from their nocturnal radiations is 

 nothing, or less decided than in open places, as we may easily 

 prove moreover directly by aid of the thermometer. 



It would be needless to add, that the influence of the clouds 

 on dew, and the cold which precedes and accompanies it, is per- 

 fectly analogous to that of trees, or of any other obstacle which 

 intercepts more or less the view of the celestial vault from the 

 radiating body. The upper clouds diminish, the lower ones 

 completely destroy, the difference between the temperature of 

 plants and that of the surrounding medium, and with it the 

 gradual cooling, the increasing humidity, and the precipitation 

 of vapour. 



It is well known that the dew is less abundant on shrubs than 

 on herbaceous plants, and that scarcely any traces of this noc- 

 turnal phaenomenon are found on the summit of trees of a cer- 

 tain height. The explanation of this fact presents itself at once, 

 if we consider that, in spite of their great emissive power, the 

 leaves of lofty plants cannot become so much cooled as the grass 

 of the meadow, nor precipitate the same quantity of water : — 



1. Because they are more exposed to the action of winds than 

 the leaves of plants nearer the ground. 



2. Because the atmospheric stratum which envelopes them is 

 less moist than that in contact with the soil. 



3. Because the air which becomes cooler and condenses itself 

 around them traverses the mass of foliage, and falls to the ground 

 without the power of reascending, as in the case of the grass, 

 towards the upper leaves, or of reacting on it or sufficiently 

 lowering its temperature, and thus acquiring the degree of 

 moisture necessary to a copious precipitation of dew. 



The currents of air which descend from the top of the trees, 

 must, like eveiy other agitation of the atmosphere, disturb the ac- 

 tions and reactions between the neighbouring bodies and the me- 

 dium which surrounds them, and thus render less intense the degree 

 of cold which these bodies would acquire in a calm atmosphere. 

 Consequently the grass situated close to trees will be less cooled 

 and less moistened by dew than that which is in the middle of 



