I need hardly say that I do not agree with Captain Maury, whose theory of oceanic circulation appears to me to be wholly untenable. Nor do I for a moment assert that the winds play no part in producing oceanic circulation. I may have been mistaken in attaching so much weight as I have to Maury's evidence as to the trade wind zones, though it is known that science owes more to him than to any man for our present knowledge of the winds prevalent in certain regions; and when I first wrote on the Gulf Stream there was no evidence on the subject even approaching Maury's (or that collected by Maury) in accuracy and completeness. But there is one argument which those who have adopted the trade winds as the primary cause of the Gulf Stream appear to me to have overlooked, and it is on this argument that my own view has been chiefly based. The trade wind zone of the northern hemisphere is not constant in position; but travels northwards and southwards with the northerly and southerly motion of the sun in declination. The change in the position of the zone of calms is not, indeed, so great as is stated in Buchan's meteorology, where it is said to travel from 25° north to 25° south of the equator; but it is considerably greater than was supposed by Dove, Kaemtz, and others. If we set the extreme shift of the northern trade-zone at ten degrees we are certainly not over-rating it. Taking this zone as extending in spring or autumn from 10° to 25° north latitude, we should have it in winter extending from 5° to 20°, and in summer from 15° to 30°, the only part common to these two ranges being that from 15° to 20° - that is to say, the northern five degrees of the winter zone, and the southern five degrees of the summer zone, each zone being 15° wide. Now, if any one will mark these zones on the North Atlantic, he will find that while the zone of winter trades would produce a current flowing into the southern half of the Gulf of Mexico, the zone of summer trades would produce a current flowing into the northern half. The former would produce a current flowing as the Gulf Stream actually flows; the latter would produce a current flowing precisely in the opposite direction. This being the case, I do not find the evidence for the trade winds as the sole or even the main cause of the Gulf Stream altogether convincing. The case does not, for instance, seem quite 'as clear as the rotation of the earth.' It seems, also, not undesirable to mention that the equatorial current and the Gulf Stream are not mere drift-currents, and that on a careful estimation of the frictional action of such winds as the trades on the surface of the ocean, the action will be found quite unequal to the propulsion of so vast a body of water as is actually carried westwards (not, by the way, before these winds). Until difficulties such as these have been removed from the trade wind theory as solely sufficient to account for the Gulf Stream, I think I would rather be the only student of science opposing that theory, than one of a phalanx, however large, maintaining it. There is, however, no such phalanx; the subject being regarded by nearly all students of science as a very open one.
English Mechanic, Aug. 30, 1872.