Sect. III.] 



HYDROGRAPHY. 



57 



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year, and a comparison of these with the actual tempera- 

 ture of the surface at any particular spot, and in any 

 particular month, would at once manifest an abnormal 

 difference, if any existed, and lead to a knowledge of its 

 Cciuse, which might prove of considerable use to the 

 mariner by acquainting him with the movement of the 



great body of water in which he was sailing ; either re- 



tarding or accelerating his progress as the case might be, 

 and at all events affecting his reckoning. Or it might 

 lead to a closer determination of the limits and periodical 

 changes of currents which, as before observed, are every- 

 where running over the surface of the sea as rivers run 

 over dry land. 



It is therefore recommended to add to the table of 

 currents a column for the temperature of the open air, 

 and another for that of the surface of the sea, which 

 should be registered frequently during the twenty- four 

 h<^urs ; but as such observations form an essential feature 

 in the meteorological register of a voyage, they should be 

 made at the times and in the manner indicated under the 

 head of Meteorology.* 



4 There should also be noted in the Eeixiark column 

 the occurrence of masses of sea-weed, or of any continued 



any 



other 



appearance even of small patches of this or -; 

 floating substances which may be seen ; and if opportunit} 

 offers, deep-sea soundings should be tried at the spot. 

 " It were much to be wished,'' says Humboldt, ' Person. 



* If pasdng Cnpe Horn, or through seas where icebergs may be 

 moving about, these observations cannot be made too frequently in thick 

 weather, especiaUy as a precaution, for the water appears to be influenced 

 to a considera'' . distance around these masses, particularly in their 



wake. 



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