_A Confusion of Ideas in Reference to the supposed Agency of Polar Cold._—It seems to me that Dr. Carpenter has been somewhat misled by a slight confusion of ideas in reference to the supposed agency of polar cold. This is brought out forcibly in the following passage from his memoir in the Proceedings of the Royal Geographical Society, vol. xv. “Mr. Croll, in arguing against the doctrine of a general oceanic circulation sustained by difference of temperature, and _justly maintaining_ that such a circulation cannot be produced by the application of heat at the surface, has entirely ignored the agency of cold.” It is here supposed that there are two agents at work in the production of the general oceanic circulation. The one agent is _heat_, acting at the equatorial regions; and the other agent is _cold_, acting at the polar regions. It is supposed that the agency of cold is far more powerful than that of heat. In fact so trifling is the agency of equatorial heat in comparison with that of polar cold that it may be “practically disregarded”—left out of account altogether,—polar cold being the _primum mobile_ of the circulation. It is supposed also that I have considered the efficiency of one of the agents, viz., heat, and found it totally inadequate to produce the circulation in question; and it is admitted also that my conclusions are perfectly correct. But then I am supposed to have left out of account the other agent, viz., polar cold, the only agent possessing real potency. Had I taken into account polar cold, it is supposed that I should have found at once a cause perfectly adequate to produce the required effect. This is a fair statement of Dr. Carpenter’s views on the subject; I am unable, at least, to attach any other meaning to his words. And I have no doubt they are also the views which have been adopted by those who have accepted his theory. It must be sufficiently evident from what has already been stated, that the notion of there being two separate agents at work producing circulation, namely heat and cold, the one of which is assumed to have much more potency than the other, is not only opposed to the views entertained by physicists, but is also wholly irreconcilable with the ordinary principles of mechanics. But more than this, if we analyze the subject a little so as to remove some of the confusion of ideas which besets it, we shall find that these views are irreconcilable with even Dr. Carpenter’s own explanation of the cause of the general oceanic circulation.