Hydrograplvy^ and the Art of Navigativn. 399 



atmosphere. It is doubtless unnecessary for us to demonstrate 

 the utility that would attach to such experiments, if they were 

 repeated on a very high mountain, such as Mowna-Roa or 

 Mowna-Kaah in the Sandwich Islands. 



Examination of an Anomaly which Atmospheric Tempera^ 

 tures, taken at different elevations, present in tlie night, when the 

 sky is calm and clear. — The temperature of atmospheric strata 

 diminishes in proportion as these strata become more elevated. 

 There is only one exception to this rule, and that is observed in 

 the night during a calm and clear state of the air. In these cir- 

 cumstances, an increasing progression takes place, to a certain 

 height. According to the experiments of Pictet, to whom we 

 owe the discovery of this anomaly, a thermometer then suspend- 

 ed in the air at two yards from the ground may indicate through- 

 out the night from 3^° to 5^° Fahr. less than a thermometer simi- 

 larly suspended in the air, but fifteen or sixteen yards higher. 



If it be recollected that solid bodies placed on the surface of 

 the ground, pass by means of radiation under a clear sky, to a 

 temperature much below that of the surrounding air, it will not 

 be denied that this air must at length be affected, by means of 

 contact, with the same coldness, and in a greater degree, accord- 

 ing as it is nearer the earth. In this, therefore, we find a plau- 

 sible explanation of the curious fact made known by the natu- 

 ral philosopher of Geneva. Our navigators will impart to it the 

 character of a demonstration, if they repeat Pictet''s experiment 

 in the open sea, by comparing, during a clear and tranquil night, 

 a thermometer placed on the deck with another attached to the 

 mast-head. Not that the superficial stratum of the ocean does 

 not experience the same effects of nocturnal radiation, in the 

 same manner as down, wool, grass, &c. ; but after its tempera- 

 ture has diminished, this bed of stratum is precipitated, be- 

 cause its specific density has become greater than that of the in- 

 ferior liquid beds. We are not, therefore, to expect in this case, 

 the enormous local colds observed by Wells in certain bodies 

 placed on the surface of the earth, nor the anomalous cold- 

 ness of the inferior air, which seems to be the consequence of 

 them. Every thing, indeed, leads to the belief, that the increase 

 ing progression of atmospheric temperature noticed on land, 

 does not exist in the open sea ; and that there the thermometer 



