182 Prof. Heunessy on Terrestrial Climate as 



The solar heat which is thus received by the ground may there- 

 fore be considered as confined ahnost entirely to a thin super- 

 ficial stratum. The air in contact with the soil becomes heated, 

 expands, and tends to ascend : a circulation thus follows be- 

 tween the upper and lower strata of the atmosphere situated 

 above the heated ground. During the night a different process 

 takes place ; for then the radiation of the soil causes its tempera- 

 ture to fall below that of the superincumbent air ; the coldest 

 stratum of the lower portions of the atmosphere being in contact 

 with the ground, the equilibrium of those above is not so much 

 disturbed. Yet, even in this case, causes exist which tend to 

 produce a series of actions and reactions between the upper and 

 lower strata of air, by which a process of convection will be ulti- 

 mately developed. These actions will be rendered especially re- 

 markable if the soil is not bare, but covered with vegetation in 

 the manner of the greater part of the dry land. This question 

 has been fully treated by Melloni*, in his memoir on the noc- 

 turnal cooling of bodies. His general proposition, that " a body 

 exposed during the night to the influence of a sky of equal clear- 

 ness and calmness, is always cooled to the same extent, what- 

 ever may be the temperature of the air," is fruitful in important 

 results. Thus is explained the great differences between the 

 temperature of the day and night on land in the torrid zone. 

 The intense cold observed during the night by Denham in tra- 

 versing the great Desert of Sahara, the process of artificial freez- 

 ing at Bengal, and the rain-like dews observed by Humboldt in 

 the forests of South America, are all necessary consequences of 

 the energy of the actions and reactions by which the outer coat- 

 ing of the earth loses the warmth it has acquired from sunshine 

 during the day. Convei'sely, the almost constant temperature 

 of the sea in tropical regions, by day and night, and the nearly 

 total absence of dew on the rigging of vessels far removed from 

 the land, clearly show the peculiar retentiveness of heat pos- 

 sessed by the water, and that, unlike the land, it does not readily 

 part with whatever warmth it may have acquired from sunshine 

 dui-ing the day. The cold southerly breezes sometimes observed 

 in Egypt t during the winter months, when the air has passed 

 over immense surfaces of sandy desert, present a striking con- 

 trast to the south-westerly winds which at the same season tra- 

 verse the ocean and visit our shores. It appears, from a commu- 

 nication in the Tiiiies newspaper, dated Melbourne, November 15, 

 1858, that in South Australia, the coldest winds during the 

 winter months are those blowing fi'om the northerly and tro- 



* Taylor's Scientific Memoirs, vol. v. pp. 453 and 530; and Annates 

 de Chimie et de Physique for February and April 1848. 

 t Kaenitz, Meteorologie, French edit. p. 45. 



