DROPPING FROM LEAVES. 33 



though not a drop of rain may be seen to fall beyond it. The 

 phenomena may be similar. 



The temperature of the human body, as ascertained by a thermo- 

 meter with its bulb placed in the shoulder-pit or under the tongue, 

 is within a very limited range — when the man is in a state of health— 

 the same in the Arctic regions and in the tropics, and the degree 

 known as blood-heat is as stable as is the boiling point of water. 

 Scarcely less stable is the freezing point of water or the freezing 

 point of mercury. It is something similar with trees. According to 

 experiments made by Megusoher, in Lombardy, and results of these 

 recorded in Memoria sur Boschi della Lombardia, and cited by Marsh, 

 trees maintain a,tall seasons a constant mean temperature of 54° Fahr. 



Observations which have been recorded by others have made it 

 appear questionable whether a tree does resist the cooling process 

 induced by external cold, but I know not of any question having been 

 raised in regard to its counteracting the effects of external heat of 

 a higher temperature than its own. This seems to be generally 

 admitted. In the animal this resistance is effected by evaporation; 

 in the vegetable it is probable it is effected by the same means, or by 

 evaporation and radiation combined. The subject will again come 

 under consideration, when it may be discussed at greater length. 



I have called attention to the dew and hoar-frost being seen on 

 the grass and herbage at early morn. In hygrometry mention is 

 frequently made of the dew-point : this is the temperature at which 

 the air is exactly saturated with the quantity of moisture it contains, 

 any fall from which would occasion a deposit of all that was in excess 

 of what exactly saturates the air at the lower temperature. In 

 accordance with observation, and with reasoning on the subject, the 

 dew-point is frequently the same as the lowest temperature of the 

 night; and the lower temperature of the grass and herbage, con- 

 sequent on radiation from their surface, occasions a deposit of dew- 

 drops in the same way as a glass of cold water, by lowering the 

 temperature of the air adjacent to it, is in like manner bedewed. 



According to the observations of Meguscher, whenever the temper, 

 ature of the air is above 67° degrees Fahr., the temperature of the 

 tree wiU be, to the extent of the excess, lower. If the temperature of 

 the atmosphere be 90°, and the dew-point 75°, there wiU be a copious 

 deposit of dew ; and if the lower temperature be the consequence of 

 radiation the deposit may be expected to take place over the whole of 

 the upper surface of the leaves, these in the aggregate, according to 



