548 MELLONI ON THE NOCTURNAL COOLING OF BODIES. 



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 mountains, ravines, lakes, meadows, 

 marshes, or running streams. The borders of Egypt, of the 

 Red Sea, of the Persian Gulf, of Chili and of Bengal, are cele- 

 brated for the richness of their dews ; (See the Voyages de Volney, 

 t. i. p. 51 ; of Burckhardt, p. 423 ; of Niebuhr, p. 10 ; of Ker 

 Porter, t.ii. p. 123 ; of Le Gentil, t. i. p. 624 ; of Ruppel, p. 186.) 

 the deserts of Central Africa, and the interior provinces of Bahia, 

 of Fernambouc, Urmia and Mazandecan, in Brazil and Persia, by 

 the almost total absence of this nocturnal phaenomenon. (Voyages 

 of Spix and Martins, t. ii. p. 624 ; of Olivier in Persia, t. i. pp. 

 123 and 145 ; of Ker Porter, t. ii. pp. 63 and 69.) 



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 water, 

 whilst still at a distance of some miles from the borders of this 

 river concealed by the land. (Olivier, t. ii. p. 225.) And Major 

 Denham says, that independently of the suffocating heat and of 

 the intense cold that he endured during the night in his memo- 

 rable journey across the Sahara, he also suffered from the extreme 

 dryness of the air until he reached a certain distance from 

 Ischad, 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. (Denham, Narrative, p. 49.) 



With regard to the intense cold experienced by this intrepid 

 traveller 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 perspiration, 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, so well characterized by the Arabs under the 

 name of the sea loithout water {el bdar billa mda), presents no 

 cause capable of disturbing during the night the equilibrium of 



