sources of heat than those noticed by me; but the omission does not diminish the importance of those to which I have referred. Had I been writing a treatise on meteorology, I should no doubt have referred as well to the influence of aqueous vapour as to many other sources of heat which I have purposely omitted in my paper on Ocean-currents as being foreign to my inquiry. Dr. Carpenter objects to my statement that "the greater part of the moisture received at the equator is condensed and falls as rain in those regions," and refers me to the case of the Red Sea, where, although evaporation is excessive, almost no rain falls. But is it not an established fact, that the greater part of the water evaporated in intertropical regions does actually fall as rain in those regions? The reason why the vapour raised from the Red Sea does not fall in that region as rain, is no doubt owing to the fact that this sea is only a narrow strip of water in a dry and parched land, the air overhead being too greedy of moisture to admit of the vapour being deposited as rain. But over a wide expanse of ocean, where the air above is kept to a great extent in a constant state of saturation, the case is totally different. I continue my quotation :-" Until corrected by Mr. Findlay, Mr. Croll assumed that the whole of the true Gulf-stream continues to flow in a N.E. direction; whereas it is unquestionable that a considerable proportion of it (probably more than one half) turns southwards to the east of the Azores, and reenters the equatorial current" (§ 104). I am not aware of having advanced any thing which could lead Dr. Carpenter or any one else to suppose that I was of opinion that the whole of the Gulf-stream flows in a N.E. direction, or that I was ignorant of the existence of the S.E. branch. Nor do I remember having seen the correction by Mr. Findlay to which he refers. To suppose that I knew of the existence of the N.E. but not of the S.E. branch, is to assume that I had never seen a chart of the Gulf-stream. If I had seen a chart, how could I possibly have observed the N.E. branch without at the same time perceiving the other? It would be just as possible to look one's friend in the face and notice his left eye without seeing his right. In reference to these four reasons or arguments designed to show that my figures are valueless, there must be some confusion of ideas. The point to be proved is, that by some wrong method I have been led to form either an erroneous estimate of the quantity of heat conveyed by the Gulf-stream, or the effects resulting from that heat. This is what Dr. Carpenter proposes to do; but no sooner does he make this proposal than he com mences to prove something totally different, viz. that there are a great many important causes affecting climate which I entirely ignore or overlook, and that those causes which I ignore or overlook have a far greater influence on climate than the heat of the Gulf-stream. It may be perfectly true that there are a great many important causes affecting climate which I have not considered; and it may likewise be true that those causes, left out of consideration, have a far more powerful influence on climate than the heat conveyed by the Gulf-stream; nevertheless it may be true also that all my statements regarding the influence of the Gulf-stream on climate are perfectly correct. It does not necessarily follow that a horse may not possess a certain amount of strength, and be able to perform a certain amount of work, simply because there are other horses which possess a much greater amount of strength, and can perform a much greater amount of work. -- What I have endeavoured to prove in reference to the Gulfstream is that the amount of heat conveyed by it is so enormous as to be equal to one fourth of all the heat received from the sun by the North Atlantic in temperate regions; and that were it not for the Gulf-stream and other ocean-currents, only a very small portion of the globe would be suited to the present orders of sentient beings that London, instead of possessing a mean annual temperature of nearly 50°, would have a mean temperature of not over 10°. But I never argued that there were not other causes to which we are far more indebted than to the heat of the Gulf-stream. Were it not for those other causes, the temperature of London would not be simply 40°, but upwards of 500° below what it is at present. The bearing which my estimate has on Dr. Carpenter's Theory. There is one point to which I wish to direct special attention, viz. the bearing which my conclusions regarding the quantity of heat conveyed by the Gulf-stream have on Dr. Carpenter's theory of a general interchange of equatorial and polar water. But, in order better to understand this matter, it will be necessary to refer very briefly to a point which has already been discussed at considerable length in former papers. In my earlier paper on the amount of heat conveyed by the Gulf-stream †, I estimated the volume of that stream as equal to that of a current 50 miles broad and 1000 feet deep, flowing (from the surface to the bottom) at 4 miles an hour. Of course I did not mean, as Dr. Carpenter seems to suppose, that the stream at any par* Phil. Mag. for Feb. 1870. + Trans. of Geol. Soc. of Glasgow for April 1867; Phil. Mag. for June 1867 (Supplement). ticular place is 50 miles broad and 1000 feet deep, or that it actually flows at the uniform rate of 4 miles an hour at surface and bottom. All I meant was, that the Gulf-stream is equal to that of a current of the above size and velocity. But in my recent papers on Ocean-currents I have taken the volume of the stream at one half this estimate, viz. equal to a current 50 miles broad and 1000 feet deep flowing at the rate of 2 miles an hour. I have estimated the mean temperature of the stream as it passes the Straits of Florida to be 65°, and have supposed that the water in its course becomes ultimately cooled down on an average to 40°*. In this case each pound of water conveys 25 units of heat from the Gulf of Mexico, to be employed in warming temperate and polar regions. Assuming these data to be correct, it follows that the amount of heat transferred from the Gulf of Mexico by this stream per day amounts to 77,479,650,000,000,000,000 foot-pounds. This enormous quantity of heat is equal to one fourth of all that is received from the sun by the whole of the Atlantic Ocean from the tropic of Cancer up to the Arctic Circle. This is the amount of heat conveyed from intertropical to temperate and polar regions by the Gulf-stream. What now is the amount conveyed by means of the general oceanic circulation? If this general interchange of equatorial and polar water be, as Dr. Carpenter supposes, the great agency employed in distributing heat over the globe, then surely it is not too much to expect that the quantity of intertropical heat carried into the North Atlantic and Arctic seas must be at least equal to that carried by the Gulf-stream. If we assume this to be the case, then the combined amount of heat conveyed by the two agencies into the Atlantic from intertropical regions will of course be equal to twice that conveyed by the Gulf-stream alone. Taking the annual quantity of heat received from the sun per unit surface at the equator at 1000, the quantities received by the three zones will be respectively as follows:— It is probable that a large proportion of the water constituting the south-eastern branch of the Gulf-stream is never cooled down to 40°; but, on the other hand, the north-eastern branch, which passes into the Arctic regions, will be cooled far below 40°, probably below 30°. Hence I cannot be overestimating the extent to which the water of the Gulfstream is cooled down in fixing upon 40° as the average minimum temperature. Now it will be seen, by referring to what has been shown on a former occasion (Phil. Mag. for Oct. 1870, p. 257), that the Gulf-stream and general oceanic circulation would, in such a case, remove from the torrid zone 405 parts of the 975 received from the sun; and this transferred to the Atlantic in temperate regions, would add 367 to the 757 already possessed by it. In this case the Atlantic in temperate regions would possess 1126 parts of heat, whereas the intertropical region would possess only 570 parts; or, in other words, the Atlantic in temperate regions would have twice the amount of heat possessed by it in intertropical regions. But if we assume that one half of this heat goes into the Arctic Ocean, and the other half remains in the temperate regions, the relative quantities of heat possessed by the three zones will be as follows: It is here assumed, however, that none of the heat possessed by the Gulf-stream is derived from the southern hemisphere, which, we know, is not the case. But supposing that as much as one half of the heat possessed by the stream came from the southern hemisphere, and that the other half was obtained from the seas lying between the equator and the tropic of Cancer, the relative proportions of heat possessed by the three zones per given area would be as follows: in Temperate zone 671 940 766 This proves incontestably that, supposing there is such a circulation as Dr. Carpenter maintains, the quantity of heat conveyed by means of it into the North Atlantic and Arctic Oceans must be trifling in comparison with that conveyed by the Gulfstream; for if it nearly equalled that conveyed by the Gulfstream, then not only the North Atlantic in temperate regions, but even the Arctic Ocean itself would be much warmer than the intertropical seas. In fact, so far as the distribution of heat over the globe is concerned, it is a matter of indifference whether there really is or is not such a thing as this general oceanic circulation. The enormous amount of heat conveyed by the Gulf-stream alone puts it beyond all doubt that oceancurrents are the great agents employed in distributing over the globe the excess of heat received by the sea in intertropical regions. It is therefore, so far as concerns the theory of a General Oceanic Circulation, of the utmost importance that the advo cates of that theory should prove that I have overestimated the thermal power of the Gulf-stream. This, however, can only be done by detecting some error either in my computation or in the data on which it is based; yet neither Dr. Carpenter nor any one else, as far as I know, has challenged the accuracy of my figures. The question at issue is the correctness of the data; but the only part of the data which can possibly admit of being questioned is my estimate of the volume and temperature of the stream. Dr. Carpenter, however, does not maintain that I have overestimated the temperature of the stream; on the contrary, he affirms that I have really underestimated it. "If we assume," remarks Dr. Carpenter, "the limit of the stratum above 60° as that of the real Gulf-stream current, we shall find its average temperature to be somewhat higher than it has been stated by Mr. Croll, who seems to have taken 65° as the average of the water flowing through the entire channel. The average surface-temperature of the Florida channel for the whole year is 80°; and we may fairly set the average of the entire outgoing stream, down to the plane of 60°, at 70°, instead of 65° as estimated by Mr. Croll" (§ 141). It follows, then, that every pound of water of the Gulf-stream actually conveys 5 units of heat more than I have estimated it to do the amount conveyed being 30 units instead of 25 units as estimated by me. Consequently, if the Gulf-stream be equal to that of a current of merely 41 miles broad and 1000 feet deep, flowing at the rate of 2 miles an hour, it will still convey the estimated quantity of heat. But this estimate of the volume of the stream, let it be observed, scarcely exceeds one third of that given by Herschel, Maury, and Colding (Phil. Mag. Oct. 1871, p. 272), and little more than one half that assigned to it by Mr. Laughton, and but very little exceeds that given by Mr. Findlay, an author whom few will consider likely to overrate either the volume or heating-power of the stream. The important results obtained during the 'Challenger' expedition have clearly proved that I have neither overestimated the temperature nor the volume of the Gulf-stream. Between Bermuda and Sandy Hook the stream is 60 miles broad and 600 feet deep, with a maximum velocity of from 3 to 4 miles an hour. If the mean velocity of the entire section amounts to 2 miles an hour, which it probably does, the volume of the stream must equal that given in my estimate t. But we have no Mr. Findlay considers that the daily discharge does not exceed 333 cubic miles (Brit. Assoc. Rep. 1869, p. 160). My estimate makes it 378 cubic miles. Mr. Laughton's estimate is 630 cubic miles (Paper "On Ocean-currents," Journ. of Royal United-Service Institution, vol. xv.). Dr. Carpenter states (§ 140) that 48 miles per day is the mean |