34: rnrsiCAL GEOGiiArnY of the sea, and its meteohology. 



see actually preserving siicli a system of counter-currents, hold, 

 at least in some degree, the relation of the supposed water and 

 oil? 



97. Warm currents flow towards tJie pole^ cold towards the equator, 

 — In obedience to the laws here hinted at, there is a constant 

 tendency (Plate IX.) of polar waters towards the tropics and of 

 tropical waters towards the poles. Captain Wilkes, of the United 

 States' Exploring Expedition, crossed one of these hyperborean 

 under-currents two hundred miles ia breadth at the equator. 



98. Edges of the Gulf Stream a striking feature. — No feature of 

 the Gulf Stream excites remark among seamen more frequently 

 than the sharpness of its edges, particularly along its inner 

 borders. There, it is a streak on the water. As high up as the 

 Carolinas this streak may be seen, like a greenish edging to a 

 blue border — the bright indigo of the tropical contrasting finely 

 (§ 70) with the dirty green of the littoral waters. It is this 

 apparent reluctance of the warm waters of the stream to mix 

 with the cool of the ocean that excites wonder and calls forth 

 remark. But have we not, so to speak, a similar reluctance 

 manifested by all fluids, only upon a smaller scale, or under cir- 

 cumstances less calculated to attract attention or excite remark ? 



99. Ulmirations. — The water, hot and cold, as it is let into the 

 tub for a warm bath, generally arranges itself in layers or 

 sections, according to temperature ; it requires Tiolent stirring 

 to break them up, mix, and bring the whole to an even tempe- 

 rature. The jet of air from the blow-pipe, or of gas from the 

 burner, presents the phenomenon still more familiarly ; here we 

 have, as with the Gulf Stream, the dividing line between fluids 

 in motion and fluids at rest finely presented. There is a like 

 reluctance for mixing between streams of clear and muddy water. 

 This is very marked between the red waters of the Missouri and 

 the inky waters of the upper Mississippi ; here the waters of each 

 may be distinguished for the distance of several miles after these 

 two rivers come together. It requires force to inject, as it were, 

 the particles of one of these waters among those of the other, for 

 mere vis inertia tends to maintain in their statu quo fluids that 

 have already arranged themselves in layers, streaks, or aggre- 

 gations. 



100. Sow the water of the Gulf Stream differs from the littoral 

 waters. — In the ocean we have the continual heaving of the sea 

 and agitation of the waves to overcome this vis inertia; and the 



