1 For the sake of completeness, and also that the present essay may fairly represent my views when it was written, I leave the account of the map and of the course of the Gulf Stream unchanged here. By comparing this essay with the following, it will be noticed that only very few passages are repeated in substance.

There will be noticed also in the map three return streams, one flowing southward outside Iceland, another sweeping round the eastern shores of Greenland, and the third flowing through Baffin's Bay and Davis's Straits. The two last unite south of Davis's Straits, and flow on together to meet the first stream outside Newfoundland, whence the three flow as a single current past the shores of the United States. It is generally assumed, and in all probability justly, that these three streams are derived from the Gulf Stream, and are different branches of its returning waters.

Between the single return-stream which laves the shores of the United States and the Gulf Stream there is an unshaded space in the map. It is not to be inferred, however, that this space represents still (or rather unflowing) water. On the contrary, it is the ' debatable ground' between the opposite currents. In spring the whole of this space is occupied by the southward flowing waters of the cold return-current. In autumn the whole of the space is occupied with the waters of the Gulf Stream. Backwards and forwards over this space the rival currents are continually swaying, the period of an oscillation being one year.

In the widest part of the Atlantic Ocean-that, namely, which extends between the most westerly part of Africa and the West Indies-there is a wide expanse of waters unmoved by the flux or reflux of currents. Surrounded on every side by the circulating waters of the Central Atlantic current-system, this region remains undisturbed save by winds and the tidal wave. Accordingly its surface is covered with different forms of marine vegetation. My readers will doubtless remember the interest which the Great Sargasso Sea excited in the mind of Christopher Columbus. Oviedo termed this region the 'seaweed meadow.' 'A host of small marine animals,' says Humboldt, ' inhabit this ever-verdant mass of Fucus natans, one of the most widely-diffused of the social plants of the ocean, constantly drifted hither and thither by the tepid winds that blow across its surface.'

In the South Atlantic there is a smaller and somewhat more sharply-defined Sargasso, covered chiefly with rockweed and drift. A weedy space occurs also about the Falkland Islands, but is probably not a true Sargasso. Maury considers that the seaweed reported there probably comes from the Straits of Magellan, where it grows so thickly that steamers find great difficulty in making their way through it; for it so cumbers their paddles as to make frequent stoppages necessary.

Such is the distribution of the surface of the Atlantic Ocean. But now the question will at once suggest itself: Is the complete system of oceanic circulation exhibited on the surface ? It seems now quite certain that this question must be answered in the negative. We might, indeed, at once point to the existence of the important current which laves the shores of the United States as an answer to the question; for where can all this water find an outlet? It does not pass the Peninsula of Florida as a current; it does not cross the Gulf Stream; where, then, can it go but underneath the ocean's surface? But we have positive evidence of the existence of under-currents.

In the first place it is found that in deep-sea soundings in many parts of the ocean, far more line may be paid out without any sign of the bottom being reached than the depth of the ocean in those parts would account for. In places where it has been proved by other methods than ordinary sounding that the depth is not more than three miles, no less than ten miles of line have been paid out, being carried out so strongly that the slightest check in the paying-out apparatus has sufficed to break the sounding-line.

In the second place, it has been found possible to determine the depth at which a submarine current is flowing, and the direction in which it flows. Thus Lieuts. Walsh and Lee, in the American service, having loaded a block of wood to sinking, and let it down to different depths, had repeatedly the satisfaction of seeing the work of under-currents. 'It was wonderful, indeed,' they write, 'to see the barrega' (a float attached to the upper end of the line) 'moving off, against wind, sea, and surface current, at the rate of over one knot an hour, as was generally the case, and on one occasion, as much as one and three-quarter knots. The men in the boat could not repress exclamations of surprise, for it really appeared as if some monster of the deep had hold of the weight below, and was walking off with it.'

Lastly, we may mention that Captain Wilkes, of the United States Exploring Expedition, established the existence of a cold under-current no less than two hundred miles broad at the equator.

We may assume, then, that a complete system of circulation, vertical as well as horizontal, exists throughout the whole of the waters contained within the great Atlantic valley.

Where are we to look for the origin of this vast series of movements? The actual 'work done' in the Atlantic Ocean is so enormous - in other words, the transfer of such large volumes of water represents so enormous a force, that we might well expect to be able at once to assign the motive-power of this great machinery. For it would seem that the giant which works such wonders could not readily hide himself from our recognition.

It has not been found, however, that the solution of the problem has been so simple as was to have been anticipated.

Passing over the earlier guesses which marked the Gulf Stream as the offspring of the Mississippi River, of the sun's motion in the ecliptic (a mysterious interpretation of the phenomena), and of the tidal wave, we may remark that but two explanations of the Atlantic currents seem to merit discussion.