CHAPTEK XYII. 



§ 720-735. THE CLIMATES OF THE SEA. 



720. Theemal charts, shoAving the temperature of the surface 

 A "milky way "in of the Atlantic Ocean by actual observations made 

 the ocean. indiscriminately all over it, and at all times of the 



year, have been published by the National Observatory. The 

 isothermal lines which these charts enable us to draw, and a few 

 of which are traced on Plate IV., afford the navigator and the 

 philosopher much valuable and interesting information touching 

 the circulation of the oceanic waters, including the phenomena of 

 their cold and warm currents ; these lines disclose a thermal tide 

 in the sea, which ebbs and flows but once a year ; they also cast 

 light upon the climatology of the sea, its hyetographic peculia- 

 rities, and the climate conditions of various regions of the 

 earth ; they show that the profile of the coast-line of intertropical 

 America assists to give expression to the mild climate of Southern 

 Europe; they also increase our knowledge concerning the Gulf 

 Stream, for they enable us to mark out, for the mariner's guid- 

 ance, that " milky way " in the ocean, the waters of which teem, 

 and sparkle, and glow w^ith life and incipient organisms as they 

 flow across the Atlantic. In them are found the clusters and 

 nebulae of the ocean which stud and deck the great highw^ay of 

 ships on their voyage between the Old World and the New ; and 

 these lines assist to point out for the navigator their limits and 

 his way. They show this via ladea to have a vibratory motion 

 in the sea that calls to mind the graceful wavings of a pennon as 

 it floats gently to the breeze. Indeed, if we imagine the head 

 of the Gulf Stream to be hemmed in by the land in the Straits 

 of Bemini, and to be stationary there, and then liken the tail of 

 the Stream itself to an immense pennon floating gently in the 

 current, such a motion as such a streamer may be imagined to 

 have, very much such a motion, do my researches show the tail 

 of the Gulf Stream to have. Running between banks of cold 

 w^ater (§ 71), it is pressed now from the north, now from the south, 

 according as the great masses of sea water on either hand may 

 change or fluctuate in temperature. 



