§ 720. THE CLIMATES OF THE SEA. 381 



CHAPTER XYII. 



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



720. Thermal charts, showing the temperature of the surface 

 A "milky way" in the ^^ ^^^ Atlantic Occau by actual observations made 

 *^^'^°- indiscriminately all over it, and at all times of the 



year, have been published by the National Observatory. The iso- 

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

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

 opher much valuable and interesting information touching the cir- 

 culation 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 peculiarities, and 

 the climatic 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 guidance, that "milky 

 way" in the ocean, t'io waters of which tecni, and sparkle, and 

 glow with life and incipient organisms as they flow across the At- 

 lantic. In them are found the clusters and nebula3 of the sea 

 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 station 

 aiy there, and then liken the tail of the Stream itself to an im- 

 mcnse 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. Run- 

 ning 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. 



