22 Baron N. Schilling on the Constant Currents 



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course. The air-currents escape our observations in the upper 

 strata of the atmosphere, and those of the sea in the depths of the 

 ocean. Notwithstanding all the improvements of nautical instru- 

 ments, we still possess no means of accurately determining the 

 currents at sea. Usually the ship's reckoning (i. e. the distance 

 run in a certain direction) is proved from time to time by astro- 

 nomical determinations of latitude and longitude ; and the dif- 

 ference thus brought to light is without hesitation attributed to 

 currents, although it may often result from quite different causes. 

 Although the unsatisfactoriness of this method has long been 

 acknowledged, it is still universally retained for want of a 

 better. 



The currents of the sea and those of the atmosphere have 

 hitherto been considered apart — probably because water and air 

 are, in many respects, so very different ; but, in spite of this 

 great difference, there can be no doubt that the movements of 

 the sea and of the atmosphere, as fluids, are subject to the same 

 general laws. For, in air as well as in water, gravity is the 

 force which generates currents, because it tends to restore equi- 

 librium wherever it has been disturbed. But equilibrium is 

 disturbed only by the following three principal causes, which, 

 again, are the same for sea and air : — 



A. Alteration of the specific gravity of the water or air ; 



B. The rotation of the earth on its axis ; 



C. The attraction of the sun and moon. 



We see, then, that the currents of the sea and the air depend 

 on the same principal causes, and hence cannot well be separated 

 in the consideration of their theory ; only the following circum- 

 stances must be kept in view. 



1. The air is a highly elastic, readily expanding, gaseous 

 body, while water is almost entirely destitute of elasticity. 



2. The atmosphere is heated by the sun principally in the 

 lower strata, causing them to expand, become lighter, and, as- 

 cending, communicate their heat to the higher. The sea, on 

 the contrary, is heated by the sun's rays on its surface only to 

 a very slight depth, and, in consequence of evaporation, gives up 

 the greater part of its heat, as latent heat, to the air. 



3. In the atmosphere we mostly observe only the currents of 

 the lower strata, and pay little attention to the upper currents, 

 though the latter are often very different, both in direction and 

 velocity, from the lower. In the water, on the other hand, we 

 direct our attention mostly to the upper currents; and only 

 quite recently have temperature-determinations at greater depths 

 begun to throw a scanty light on the deeper currents of the 

 ocean. 



4. The seas are bounded by continents, which set impassable 



