Atmospherical Phoenomena. ii47 



in the shade is not more than 30° of the centigrade thermome- 

 ter (86^ Fahr.), we find that a superficial layer of dark sand has 

 acquired from the sun a temperature of 50° C. (122° Fahr.), or 

 more. If, on the other hand, the surface consists of water, all 

 the rays are not absorbed by it ; a large portion penetrates into 

 the interior of the mass, and thus the latter also, and not merely 

 the superficial layer, is heated. Hence in summer the tempe- 

 rature of water during the day is never so elevated as that of 

 the neiglibouring land. In addition to the cause mentioned, 

 there are others which arise from the nature of water. Al though 

 the uppermost layer is somewhat more strongly heated than 

 those which lie at a small depth beneath, yet when this heating 

 takes place evaporation ensues, which increases with the tem- 

 perature, and thus the uppermost portions are cooled, acquire 

 a greater density than the layers beneath, and sink down, while 

 others reach the surface, soon afterwards to sink in their turn. 

 These unceasing movements are the cause that the uppermost 

 portion of water possesses by day an uniform temperature, but 

 one that is lower than that of the surface of the solid land, and 

 this difference necessarily shews itself in the strata of air in con- 

 tact with it. 



But it is not only the various conducting powers and the dif- 

 ferent states of aggregation that are causes of the differences 

 between land and sea ; there are also vapours floating in the at- 

 mosphere whose effects must be taken into consideration. Since 

 a rapid evaporation ensues over large collections of water, the 

 atmosphere must contain a much larger quantity of vapour, 

 under such circumstances, than where it is over dry land. Were 

 the latter always in a liquid state it would exercise no influence 

 on the power of the sun's rays, nay it seems probable that a 

 mcHst atmosphere has less effect than a dry one in diminishing 

 the light of the sun. But when there is much vapour present, 

 the upper strata of the atmosphere especially are easily satu- 

 rated.; clouds are formed, which diminish the influence of the 

 sun on the ground, or masses of rain fall, which communicate 

 the lower temperature of the upper portions of the atmosphere 

 to the surface of the earth, and contribute to the cooling of the 

 latter, a phenomenon which necessarily must take place much 

 more rarely in the case of dry solid land. 



What has hitherto been said refers merely to the changes 



