382 PHYSICAL GEOGRAPHY OF THE SEA, AND ITS METEOROLOGY, 



CHAPTER XVII. 



§^720-735. — THE CLIMATES OF THE SEA. 



720. A " onilJcy way" in the ocean. — Thermal charts, showing 

 the temperature of the surface of the Atlantic Ocean by actual 

 observations made indiscriminately all over it, and at all times 

 of the year, have been published by the National Observatory. 

 The isothennal 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 with 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 highway 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 lactea 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 

 water (§ 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. 



721. TJie vibrations of the Gulf Stream. — In September, when the 

 waters in the cold regions of the north have been tempered, and 

 been made warm and light b}- the heat of summer, its limits on 



