The explanation goes further:—“It is along the surface and upper portion of the ocean that the equatorial waters flow towards the poles, and it is along the bottom and under portion of the ocean that polar waters flow towards the equator; or, in other words, the warm water keeps the _upper_ portion of the ocean and the cold water the _under_ portion.” With this explanation I to a great extent agree. It is evident that, in reference to the northern hemisphere at least, the most of the water which flows from inter-tropical to polar regions (as, for example, the Gulf-stream) keeps to the surface and upper portion of the ocean; but for reasons which I have already stated, a very large proportion of this water must return in the form of _under_ currents; or, which is the same thing, the return compensating current, whether it consist of the identical water which originally came from the equator or not, must flow towards the equator as an under current. That the cold water which is found at the bottom of the Atlantic and of inter-tropical seas must have come as under currents is perfectly obvious, because water which should come along the surface of the ocean from the polar regions would not be cold when it reached inter-tropical regions. _The Explanation hypothetical._—Here the general agreement between us in a great measure terminates, for Dr. Carpenter is not satisfied with the explanation generally adopted by the advocates of the _wind theory_, viz., that the cold water found in temperate and inter-tropical areas comes from polar regions as compensating under currents, but advances a _hypothetical_ form of circulation to account for the phenomenon. He assumes that there is a _general set_ or flow of the surface and upper portion of the ocean from the equator to polar regions, and a _general set_ or flow of the bottom and under portion of the ocean from polar regions to the equator. Mr. Ferrel (_Nature_, June 13, 1872) speaks of that “interchanging motion of the water between the equator and the pole _discovered_ by Dr. Carpenter.” In this, however, Mr. Ferrel is mistaken; for Dr. Carpenter not only makes no claim to any discovery of the kind, but distinctly admits that none such has yet been made. Although in some of his papers he speaks of a “_set_ of warm surface-water in the southern oceans toward the Antarctic pole” as being well known to navigators, yet he nowhere affirms, as far as I know, that the existence of such a general oceanic circulation as he advocates has ever been directly determined from observations. This mode of circulation is _simply inferred_ or _assumed_ in order to account for the facts referred to above. “At present,” Dr. Carpenter says, “I claim for it no higher character than that of a good working _hypothesis_ to be used as a guide in further inquiry” (§ 16); and lest there should be any misapprehension on this point, he closes his memoir thus:—“At present, as I have already said, I claim for the doctrine of