Page:Physical Geography of the Sea and its Meteorology.djvu/70

 44 common among mariners—that the coasts of the United States and the Shoals of Nantucket turn the Gulf Stream towards the east; but if the view I have been endeavouring to make clear be correct, it would appear that the course of the Gulf Stream is fixed and prescribed by exactly the same laws that require the planets to revolve in orbits, the planes of which shall pass through the centre of the sun; and that, were the Nantucket Shoals not in existence, the course of the Gulf Stream, in the main, would be exactly as it is and where it is. The Gulf Stream is bound over to the North Sea and Bay of Biscay partly for the reason, perhaps, that the waters there are lighter than those of the Mexican Gulf; and if the Shoals of Nantucket were not in existence, it could not pursue a more direct route. The Grand Banks, however, are encroaching (§ 116), and cold currents from the north come down upon it: they may, and probably do, assist now and then to turn it aside.

123. Herschel's theory not consistent with known facts.—Now if this explanation as to the course of the Gulf Stream and its eastward tendency hold good, a current setting from the north towards the south should (§ 103) have a westward tendency. It should also move in a circle of trajection, or such as would be described by a trajectile moving through the air without resistance and for a great distance. Accordingly, and in obedience to the propelling powers derived from the rate at which different parallels are whirled around in diurnal motion (§ 91), we find the current from the north, which meets the Gulf Stream on the Grand Banks (Plate IX.), taking a south-westwardly direction, as already described (§114). It runs down to the tropics by the side of the Gulf Stream, and stretches as far to the west as our own shores will allow. Yet, in the face of these facts, and in spite of this force, both Major Rennell and M. Arago would make the coasts of the United States and the Shoals of Nantucket to turn the Gulf Stream towards the east: and Sir John Herschel (§79) makes the trade-winds, which blow from the eastward, drive this stream to the eastward!

124. The Channel of the Gulf Stream shifts with the season.—But there are other forces operating upon the Gulf Stream. They are derived (§ 80) from the effect of changes in the waters of the whole ocean, as produced by changes in their temperature and saltness from time to time. As the Gulf Stream leaves the coasts of the United States, it begins to vary its position according to