178 ANNUAL OF SCIENTIFIC DISCOVERY. 



the feeble degree of cold acquired by substances freely exposed on 

 the deck, totally unable as it is to condense the vapor of the sea atmo- 

 sphere, is nevertheless sufficient for that of the air which has been in 

 nocturnal contact with the soil. 



Dew becomes more abundant as"we approach the equator. We con- 

 clude that dew, feeble or non-existent towards the poles, by reason of 

 the extreme brevity of the summer nights, becomee more and more 

 abundant as we approach the equator ; that, notwithstanding the gen- 

 eral course of the phenomena is very much modified by the 

 extent, the nature, and the position of the land, according as it is 

 more or less surrounded by the sea, more or less covered by moun- 

 tains, lakes, meadows, marches, and running streams. The borders of 

 Egypt, of the Red Sea, of th3 Persian Gulf, of Chili, and of Bengal, 

 are celebrated for the richness of their dews ; the deserts of Central 

 Africa, and the interior provinces of Bahia, Brazil, and Persia, by the 

 almost total absence of this nocturnal phenomenon. 



The appearance of dew may serve in certain cases to make known the 

 proximity of a mass of water concealed from the eye of the observer. 

 Thus, the dew which is almost completely wanting in certain sterile 

 valleys traversed by the Euphrates, becomes of sufficient intensity to 

 form visible drops of waters, whilst at a distance of some miles from 

 the borders of this river, concealed by the land. And Mr. Denham 

 says, that independently of the suffocating heat, and of the intense 

 cold that he endured during the night, in his memorable journey 

 across the Sahara, he also suffered from the extreme, dryness of the air, 

 until he reached a certain distance from Lake Tchad, where, 

 though there was not the slightest appearance of water on any part 

 of the horizon, the dews began to appear, feeble at first, then more 

 and more copious, and so abundant on arriving near the banks of this 

 great African lake, that the clothes of those persons who remained 

 some time outside the tents were completely soaked with it. 



With regard to the intense cold experienced by Denham, during 

 the night in the desert, it is occasioned (in my opinion) neither by 

 the extreme clearness of the sky, nor by an excess of cutaneous per- 

 spiration, but from the great nocturnal calm of this desolate region, 

 which allows the soil to act strongly on the air, and to receive, with 

 equal force, the reaction of that fluid. Observe, first, that a dry, flat, 

 monotonous, horizontal, and uniformly extended country, like this 

 immense plain of Northern Africa, presents no cause capable of dis- 

 turbing, during the night, the equilibrium of the air ; so that this must 

 remain in a state of absolute rest some time after the setting of the 

 sun ; the soil of the desert being, moreover, composed of dry, sandy 

 earths, of bad conducting quality, can receive from the interior but a 

 very poor compensation in exchange for the heat it has lost. The 

 solid body radiating by night towards space and the surrounding 

 medium, will therefore be unmoving and isolated, and thus be in 

 highly favorable conditions for reacting with energy on each other, 

 and considerably lowering their temperature. 



Another phenomenon resulting from the combination of the two 



