see actually preserving such a system of counter-currents, hold, at least in some degree, the relation of the supposed water and oil?
97. Warm currents flow towards the 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 in 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 circumstances less calculated to attract attention or excite remark?
99. Illustrations.—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 violent stirring to break them up, mix, and bring the whole to an even temperature. 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 aggregations.
100. How 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
