with Geographic Botany. — Humidity. 329 



In speaking of the effects of cultivation on the temperature, 

 its influence on the production of rain has been in some mea- 

 sure anticipated ; wherever large tracts have been cleared of 

 forests, as in the United States, some parts of the Canadas, 

 and the West India islands, the quantity of rain has mate- 

 rially diminished. From two causes vegetation favours the 

 formation of rain, by supplying an abundance of moisture, 

 and increasing the daily range in the temperature ; as an in- 

 stance of the increase of rain, the island of Ascension has 

 been mentioned, where the introduction of a trifling extent of 

 cultivation has perceptibly increased the deposition. 



2. That depression of temperature which gives rise to the 

 formation of dew is produced by terrestrial radiation ; sub- 

 stances radiate with varying intensity according to their struc- 

 ture and colours, and of all, none has been found to radiate 

 so freely as the green parts of plants ; they are hence well 

 provided with the means of supplying themselves with moist- 

 ure. Dews are of the greatest importance to vegetation, 

 particularly in those situations where rain is rare, or falls 

 copiously only in one season ; during the dry seasons of low 

 latitudes, it is to them that vegetation is indebted chiefly for 

 moisture. Whilst the sun is absent plants imbibe it freely, 

 and by this change of action a period of repose is allowed to 

 vegetation, which may be compared to a man who gives rest 

 to one set of muscles by putting another into activity. 



In our climate the amount of moisture which falls as dew 

 has been estimated as equal to five inches, and the quantity 

 is considerably greater in lower parallels. The excess of de- 

 position occurs in those climates which have long dry seasons, 

 and, judging from appearances, the quantity here must be 

 very great. Its good effects on vegetation are not entirely in 

 proportion to the quantity, but more to the gradual and per- 

 fect manner in which small portions are applied at intervals 

 to the organs after the exhaustion of a burning sun. In the 

 morning, when the full nocturnal amount has been precipi- 

 tated, every substance is covered with pearls of moisture ; the 

 trees drip as after a heavy shower ; so completely is the dusty 

 surface of the roads moistened, that a water-cart seems to 

 have passed over them, and all the smaller vegetation is laden 

 with drops of dew. As the sun rises all this soon vanishes ; 

 but whether resumed by the atmosphere again, or, aided in 

 energy by the light and heat, the vegetation absorbs it, is, I 

 think, doubtful. Dew commences to form soon after sunset, 

 and sometimes, when the air is well saturated, even before ; 

 it continues progressively through the night, accompanying 

 the gradual diminution of temperature, and occasionally till 

 Ann. ^ May. N. Hist. Vol. ix. Z 



