370 M. Melloni m V 



ew. 



Presence of Dew makes known the proximity of Masses of Water 

 concealed from the Eye. 



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 steril valleys traversed by 

 the Euphrates, becomes of sufficient intensity to form visible 

 drops of water, whilst at a distance of some miles from the 

 borders of this river, concealed by the land (Oliver., 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 memorable 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 sometime outside the tents were 

 completely soaked with it. — (Denham, Narrative, p. 49.) 



Intense Cold during the Night in the Great Desert. 

 With regard to the intense cold experienced by this in- 

 trepid traveller, 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 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 

 equalforce 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 charac- 

 terised by the Arabs under the name of the Sea ivithout 

 water {El baar billa >naa\ presents no cause capable of dis- 

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

 this must remain in a state of almost 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 com- 



