Sir John Herschel is the principal exponent of the first theory, which assigns to the trade-winds the principalalmost the sole-agency in the generation of the Atlantic current-system. He refuses indeed, to look on the subject as one of any doubt or difficulty. 'The dynamics of the Gulf Stream have of late,' he writes, 'been made a subject of much (we cannot but think misplaced) wonder, as if there could be any possible ground for doubting that it owes its origin entirely to the trade-winds.' ' If there were no atmosphere, there would be no Gulf Stream, or any other considerable oceanic current (as distinguished from a mere surface-drift) whatever.' He presents his solution somewhat as follows: - The trade-winds are an actually existent cause for an easterly motion in the tropical seas; we cannot ignore their action; we know, also, that when the trade-winds arrive at the equator, they have lost their easterly momentum; and we know, therefore, that that momentum must have been imparted to the surface of the water (for where else can it have gone ?); hence there arises the great easterly movement which generates the whole system of circulation.
The second view, which attributes oceanic circulation to differences of temperature and saltness in different parts of the ocean, is supported by Humboldt and others, but is taken up most unflinchingly by Captain Maury, who assigns it as practically the sole cause of all oceanic circulation. 'The Gulf Stream,' he writes, 'as well as all the constant currents of the sea, is due mainly to this cause. Such differences are inconsistent with aqueous equilibrium, and to maintain this equilibrium the great currents are set in motion. The agents which derange equilibrium in the waters of the sea, by altering specific gravity, reach from the equator to the poles, and in their operations they are as ceaseless as heat and cold; consequently, they call for a system of perpetual currents to undo their perpetual work.' 'Other causes help to cause currents,' he says, ' but the currents created by them are ephemeral'
Here we have what is 'a very pretty quarrel as it stands.' Each of the disputants points to causes of acknowledged importance, and also (whether efficient or not in the particular matter under question) of acknowledged general efficiency. Each has much to say in favour of his own view, and each considers his antagonist's agent as utterly insufficient for the work ascribed to it. Each has heard his opponent's arguments, and reiterates his own statement. Nor can it be said that the opponents are unequally matched; for, if we must place Sir John Herschel far before Maury as a mathematician and physicist, and if we must undoubtedly look upon the former as the more practised reasoner, yet we must remember, in turn, the special attention which Captain Maury has given to the subject under discussion, and the practical acquaintance with it which his experience as a seaman has given to him.
Let us briefly state the arguments adduced by Herschel against Maury's view, and by Maury against Herschel's.
Sir John Herschel asserts that, inasmuch as the sun's heat warms the surface of the ocean most intensely, so that the water of least specific gravity is already uppermost, there can be no tendency to motion. For the heated water cannot descend, being buoyant; nor ascend, being uppermost; nor move laterally, having no impulse to motion of that sort, and being only able to move laterally 'by reason of a general declivity of surface, the dilated portion occupying a higher level.' He then applies to this declivity the test of quantitative analysis. Taking a column of water at the equator having at the base a temperature of 39° (at which temperature fresh water attains its greatest density, and which is also the temperature of water 7,200 feet beneath the surface at the equator), while its top has a temperature of 84° (the warmth of equatorial surface-water), he finds that such a column is 10 feet higher than a similar column in latitude 56°, where 39° is the surface temperature. And since from the equator to latitude 56° the distance is 3,360 geographical miles, we have a declivity of barely one-twenty-eighth of an inch per geographical, or one-thirty-second of an inch per statute mile. Such a declivity is utterly insufficient to account for the existence of a strong current from the equator towards the tropics; while, so far from giving any account of the motion of the equatorial current from east to west, it would tend to form a north-easterly current.
This seems strongly opposed to Maury's view, and I do not find that he does much to get over the force of Herschel's reasoning. He points out, indeed, that sea-water does not attain its greatest density at a temperature of 39°, but some 12° or 14° lower. This, however, does not affect Herschel's argument. If he had taken a column whose base had a temperature of 25° instead of 39°, he would have had to extend, also, the range of the water-slope in latitude; and, in fact, he would have obtained a yet smaller declivity in this way than that actually deduced by him. Maury does not seem to have noticed the really weak point in Herschel's argument. I shall presently show where this seems to me to lie.
But if Maury fails in efficiently defending his own views, he certainly is sufficiently effective in his attack upon Sir John Herschel's.
He asks, in the first place, the pertinent question-' How can the north-easterly trade-winds, which blow only 240 days out of 365, cause the equatorial current to flow all through the year towards the north-west without varying its velocity either to the force or to the prevalence of the trade-winds ?' ' That the winds do make currents in the sea, no one,' he says, ' will have the hardihood to deny; but currents that are born of the winds are as unstable as the winds; uncertain as to time, place, and direction, they are sporadic and ephemeral.'