CHAPTER XVII.
§ 720-735.—THE CLIMATES OF THE SEA.
720. A "milky 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 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 peculiarities, 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 guidance, 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. The 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 by the heat of summer, its limits on
