42 PHYSICAL GEOGRAPHY OF THE SEA, AND ITS METEOROLOGY. 



And tliis coui'se, as already remarked, is nearly that of the great 

 circle, and of the supposed cannon-ball. 



122. Many philosophers have expressed the opinion — indeed, 

 Shoals of xantucket the hoHef (§ 116) is common among mariners — that 

 course. the coasts of the United States and the Shoals of 

 Nantucket tm-n the Grulf Stream towards the east ; but if the view 

 I have been endeavouring to make clear be correct, it would ap- 

 pear 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 GuK Stream is boimd 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 Nan- 

 tucket were not in existence, it could not pm-sue a more direct 

 route. The Grand Banks, however, are encroaching (§ 116), and 

 cold cmTents from the north come do^vn upon it : they may, and 

 probably do, assist now and then to tm^n it aside. 



123. Now if this explanation as to the course of the Guit Stream 

 ^otTnti'^tent*iTth ^^^ ^^^ castward tendency hold good, a cm-rent set- 

 known facts, ting 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. Ac- 

 cordingly, and in obedience to the propelling powers derived 

 from the rate at which different parallels are whhled aromid in 

 diurnal motion (§ 91), we find the current from the north, which 

 meets the Gulf Stream on the Grand Banks (Plate IX.), taking a 

 Q0VLt}i-2vestivardIi/ dhection, as abeady 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 Eennell and 

 M. Arago would make the coasts of the United States and the 

 Shoals of Nantucket to tm-n the Gulf Stream towards the east : 

 and Sh John Herschel (§79) makes the trade-winds, which blow 

 from the eastward, drive this stream to the eastward ! 



124. But there are other forces operating upon the Gulf Stream. 

 The Channel of the They are derived (^ 80) from the efiect of chansfes 



Gulf stream shifts . .M , r }i i i j l l 



with the season. m the watcTs 01 the wholc ocean, as produced by 

 changes in their temperatui'e and saltness fi'om time to time. As 



