in the Air and in the Sea, 23 



limits to the currents, and thereby exert great influence on their 

 direction, extension, and velocity. It is quite otherwise with 

 the atmosphere, which encompasses the globe, and is undisturbed 

 in its free motion, save, perhaps, in some degree by lofty moun- 

 tain-ranges. But, on the other hand, currents may arise in the 

 atmosphere through the influence of the interior parts of the 

 continents, to which the sea has nothing to correspond. 



Lastly, mention should be made of a certainly only conventional 

 difference between air- and ocean-currents. This is, that the 

 winds are universally named after the point from which they 

 come, while ocean-currents always bear the name of the point 

 toward which they flow. This difference of nomenclature ap- 

 pears at first sight inconvenient; but use has so naturalized 

 these designations in all languages that, as Laughton* quite cor- 

 rectly remarks, any attempt to alter this custom would only give 

 rise to misunderstandings. To impress this upon young sailors, 

 the following phrase is used in the Russian navy : — " The wind 

 blows to the card ; currents flow from the card." 



The differences just mentioned between the air and water ex- 

 plain why the currents in the atmosphere often appear to us 

 quite different from those of the ocean, since the two classes of 

 currents are exposed to the action of so many and various colla- 

 teral circumstances that the community of their fundamental 

 laws can almost be no longer discovered. And yet we must be 

 clear about these fundamental laws before we can enter upon the 

 consideration of the collateral actions. 



The constancy with which the great ocean-currents and the 

 trade-winds move, and the analogy which prevails between them, 

 justify us in believing that they are less exposed to the opera- 

 tion of secondary causes and, therefore, are especially adapted 

 for the study of the general laws of currents. Hence we will 

 examine singly the above-named three causes, which, to our 

 knowledge, are alone capable of disturbing the equilibrium of 

 the sea and the atmosphere. We will ascertain how far each of 

 them is to be regarded as a generator of the constant sea-cur- 

 rents and the trade-winds, and in what measure it answers to 

 the existing explanations of these currents. The shifting winds 

 and smaller coast-currents we will in general leave unnoticed, 

 because (as already said) our information is still much too 

 limited for us to be able to form even the remotest idea of a 

 theory embracing all currents. 



However, before we come to the examination of the forces 

 which call forth the constant currents, we will briefly describe 

 the great oceanic currents and the trade-winds, and indicate how 



* Physical Geography (London, 18/0). j>. \~6. 



