He then points to a fact which 'militates strongly against the vast current-begetting power that has been given by theory to the gentle trade-winds. In both oceans, the Sargasso seas lie partly within the trade-wind region; but in neither do these winds give rise to any current. The weeds are partly out of water, and the wind has therefore more power upon them than it has upon the water itself; they tail to the wind. And if the supreme power over the currents of the sea reside in the winds, as Herschel would have it, then of all places in the trade-wind region, we should here have the strongest currents. Had there been currents here, these weeds would have been borne away long ago; but so far from it, we know that they have been in the Sargasso Sea of the Atlantic since the voyage of Columbus.'
In another argument, Maury certainly falls into an error. He says, How can the north-easterly winds cause the Gulf Stream to flow towards the north-east ? But, as he himself points out, the trade-winds do not blow over the Gulf Stream proper, and there can be no doubt that, if the trade-winds sufficed to keep up a continual equatorial current, finding a passage towards the north after encountering the barrier opposed by the American continent, this resulting northerly current would assume a north-easterly course, for the very same reason that the air-currents flowing from the equator towards the north pole become south-westerly or counter trade-winds. But he seems justified in asking how it is possible that the impulse imparted by the gentle trade-winds to the equatorial current could suffice to generate a stream which eventually travels far towards the north pole, if it do not even circle completely around Greenland. 'When we inject water into a pool,' he says, ' be the force never so great, the jet is soon overcome, broken up, and made to disappear. In this illustration, the Gulf Stream may be likened to the jet, and the Atlantic to the pool. We remember to have observed, as children, how soon the mill-tail loses its current in the pool below; or we may now see at any time, and on a larger scale, how soon the Niagara, current and all, is swallowed up in the lake below.'
Franklin, who was the originator of the theory supported by Herschel, had unnecessarily introduced the supposition that the trade-winds maintain a 'head of water' in the Gulf of Mexico, and that the Gulf Stream flows downwards like a river from this 'head,' as a fountain or source. Maury rightly attacks this view, which is undoubtedly a mistaken one; but in doing so, he falls into an error which exhibits his weakness in the treatment of hydrodynamical problems. He points out that, inasmuch as the Gulf Stream grows wider as it crosses the Atlantic, it necessarily grows shallower, so that the water-bed in which the stream flows has a higher level under the shallow than under the deep part of the current, and therefore, says Maury, 'the current runs up hill' Herschel terms this a strange perversion of language, but perhaps it would be more correct to speak of it as a strange blunder. The stream could, of course, only be said to run up hill if its surface were seeking a higher level, which does not and cannot happen. That the spreading out of the water of the current, so as to form a wider and shallower stream, does not correspond to an upward flow, is evident from this, that it happens often with rivers, which no one will suspect of running up hill.
Herschel does not find an answer to the main objections urged by Maury against the trade-wind theory. Content with urging an apparently unanswerable objection against his opponent's view, he leaves his own to take care of itself.
In forming an opinion respecting the two theories, one is struck with the immense superiority in the power of Maury's agent. For, if we consider, we shall see that almost the whole of the surfs action upon the ocean goes to produce those variations in temperature and saltness in which Maury sees the origin of the current-system; but a very moderate portion of the sun's action is called into play in the production of the trade-winds. Now it is very doubtful whether any large proportion even of the force expended in producing the trade-winds, ever acts on the water. For we know that the northeasterly and south-easterly air-currents of the northern and southern hemispheres, do not wholly merge into northern and southern currents meeting point-blank near the equator, as Herschel's theory seems to imply. On the contrary, there is a wide zone of calms at the equator, and the two systems of trade-winds appear to pass upwards above the calm air, without parting with the whole of their easterly motion. When once they begin to travel polewards, they lose their easterly motion in the same way that they acquired it-that is, through the effects of the earth's rotation. And whatever portion is lost in this way-which, for aught we know, may be a very considerable portion-cannot be taken into account as available to generate the easterly equatorial current.
And now let us consider for a moment the relation which holds between cause and effect in the case supposed by Herschel. We have more than a fourth part of the Atlantic Ocean in a state of perpetual motion, and it is assumed that the air immediately above the ocean is responsible for this circulation. Now even if we suppose that the whole of the vis viva in the aerial circulation is imparted to the waters, and neglect all consideration of the fact that for a large portion of the year the winds do not act in the manner available for the production of the currents we are considering, yet even then, I apprehend that we shall find the vis viva of the aerial very far below that of the aqueous circulation. The volume of moving water is, of course, far less than that of the moving air, and the mean velocity of the water-currents is less than that of the air-currents; but, on the other hand, the specific gravity of water is some 830 or 840 times greater than that of air, and this difference far more than counterbalances the others.
But now, when we come to consider the forces called into action in producing changes of temperature, etc., we no longer find such a disproportion between cause and effect. The sun's action on the equatorial and tropical regions of the Atlantic not only produces a great change in the density of the water, but also raises immense masses by evaporation. Now the buoyancy caused by increase of temperature is partly diminished through increase of saltness; still it is an important motive force. A large portion of the evaporated water is also precipitated over the equatorial regions in the form of rain; yet that a very large portion is carried away from equatorial and tropical to temperate zones is beyond dispute.
But now, how are we to get over the arguments by which Herschel seeks to show that the buoyant water will not rapidly move off, and that the effect of evapo-ration is merely to produce opposing inrushes of water which destroy each other's effect ? Easily, I take it, if we remember that the buoyancy of the water does produce a surface-flow from the equator, however slight, and that this is sufficient to destroy the balance of forces which might otherwise make it doubtful whether the place of the evaporated water would be supplied from below or from above. I apprehend that there is a continual under-flow of cooler water, rushing in towards the equator on both sides, to supply the place of the water evaporated by the sun's heat. Now there can be no question that under-currents arriving in this manner, whether from the north or from the south, would acquire a strong westerly motion (just as the trade-winds do). Thus they would generate from below the great equatorial westerly current. In this up-flow of cool currents having a strong westerly motion, I find the mainspring of the series of motions. The water thus pouring in towards the equator is withdrawn from beneath the temperate and arctic zones, so that room is continually being made for that north-easterly surfacestream which is the necessary consequence of the continual flow of the great western equatorial current against the barrier formed by the American continent. It would require much more space than I have at my disposal to deal at length with the subject of my paper. I therefore conclude by referring my readers to Maury's interesting work on the 'Physical Geography of the Sea,' with the remark that his views seem to me only to require the mainspring or starting force towards the west which I have ventured to suggest, to supply a complete, efficient, and natural explanation of the whole series of phenomena presented by the great ocean-currents.
The Student for July 1868.