that of a stream 50 miles broad and 1000 feet deep, flowing at the rate of four miles an hour. And the mean temperature of the entire mass of moving waters is not under 65° at the moment of leaving the Gulf." I think we are warranted to conclude that the Gulf-stream, before it returns from its northern journey, is on an average cooled down to at least 40°, consequently it loses 25° of heat. Each cubic foot of water, therefore, in this case carries from the tropics for distribution upwards of 1500 units of heats, or 1,158,000 foot-pounds. According to the above estimate of the size and velocity of the stream 5,575,680,000,000 cubic feet of water are conveyed from the Gulf per hour, or 133,816,320,000,000 cubic feet daily. Consequently the total quantity of heat transferred from the equatorial regions per day by the stream amounts to 154,959,300,000,000,000,000 footpounds. From observations made by Sir John Herschel and by M. Pouillet on the direct heat of the sun, it is found that were no heat absorbed by the atmosphere, about 83 foot-pounds per second would fall upon a square foot of surface placed at right angles to the sun's rays. Mr. Meech estimates that the quantity of heat cut off by the atmosphere is equal to about 22 per cent. of the total amount received from the sun. M. Pouillet estimates the loss at 24 per cent. Taking the former estimate, 64-74 foot-pounds per second will therefore be the quantity of heat falling on a square foot of the earth's surface when the sun is in the zenith. And were the sun to remain stationary in the zenith for twelve hours, 2,796,768 foot-pounds would fall upon the surface. 3 It can be shown that the total amount of heat received upon a unit-surface on the equator during the twelve hours from sunrise till sunset at the time of the equinoxes is to the total amount which would be received upon that surface, were the sun to remain in the zenith during those twelve hours, as the diameter of a circle to half its circumference, or as 1 to 1.5708. It follows, therefore, that a square foot of surface on the equator receives from the sun at the time of the equinoxes 1,780,474 foot-pounds daily, and a square mile 49,636,750,000,000 foot-pounds daily. But this amounts to only 3. part of the quantity of heat daily conveyed from the tropics by the Gulf-stream. In other words, the Gulf-stream conveys as much heat as is received from the sun by 3,121,870 square miles of surface at the equator. The amount thus conveyed is equal to all the heat which falls within 63 miles on each side of the equator. According to calculations made by Mr. Meech,' the annual quantity 3.181,870 1 Phil. Mag. for February 1867, p. 127. Captain Maury considers the Gulf-stream equal to a stream 32 miles broad and 1200 feet deep, flowing at the rate of five knots (38,415 feet) an hour (Physical Geography of the Sea, § 24). This gives 6,166,700,000,000 cubic feet per hour as the quantity of water conveyed by this stream. Sir John's Herschel's estimate is still greater. He considers it equal to a stream 30 miles broad and 2200 feet deep, flowing at the rate of four miles an hour (Physical Geography, § 54). This makes the quantity 7,359,900,000,000 cubic feet per hour. Sir John estimates the temperature at 86 F. Trans. of Royal Soc. of Edin., Vol. xxi., p. 57. Phil. Mag. S. 4, Vol. ix. p. 36. Smithsonian Contributions to Knowledge, Vol. IX. of heat received by a unit surface on the frigid zone, taking the mean of the whole zone, is of that received at the equator. Consequently the quantity of heat conveyed by the Gulf-stream in one year is equal to the heat which falls on an average on 6,873,800 square miles of the arctic regions. The frigid zone or arctic regions contain 8,130,000 square miles. There is actually, therefore, nearly as much heat transferred from the tropical regions by the Gulf-stream as is received from the sun by the entire arctic regions; the quantity conveyed by the stream to that received from the sun by those regions being as 15 to 18. But we have been assuming in our calculations that the percentage of heat absorbed by the atmosphere is no greater in polar regions than it is at the equator, which is not the case. If we make due allowance for the extra amount absorbed in polar regions in consequence of the obliqueness of the sun's rays, the total quantity of heat conveyed by the Gulf-stream will probably nearly equal the amount received from the sun by the entire arctic regions. If we compare the quantity of heat conveyed by the Gulf-stream with that conveyed by means of aerial currents, the result is equally startling. The density of air to that of water is as 1 to 770, and its specific heat to that of water is as 1 to 4.2. Consequently the same amount of heat that would raise 1 cubic foot of water 1° would raise 770 cubic feet of air 40.2, or 3234 cubic feet 1°. The quantity of heat conveyed by the Gulf-stream is therefore equal to that which would be conveyed by a current of air 3234 times the volume of the Gulf-stream, and at the same temperature and moving with the same velocity. Taking, as before, the width of the stream at 50 miles, and its depth at 1000 feet, and its velocity at 4 miles an hour, it follows that in order to convey an equal amount of heat from the tropics by means of an aërial current, it would be necessary to have a current about 1 mile deep and at the temperature of 65° blowing at the rate of four miles an hour from every part of the equator over the northern hemisphere towards the pole. If its velocity were equal to that of a good sailing-breeze, which Sir John Herschel states to be about twenty-one miles an hour, the current would require to be above 1200 feet deep. A greater quantity of heat is probably conveyed by the Gulf-stream alone from the tropical to the temperate and arctic regions than by all the aërial currents which flow from the equator. We are apt, on the other hand, to over-estimate the amount of heat conveyed from tropical regions to us by means of aërial currents. The only currents which flow from the equatorial regions are the upper currents or anti-trades, as they are called. But it is not possible that much heat can be conveyed to us directly by them. The upper currents of the trade-winds, even at the equator, are nowhere below the snow-line. They must, therefore, lie in a region actually below the freezing-point. In fact, if those currents were warm, they would elevate the snow-line above themselves. The heated air rising off the hot burning ground at the equator, after ascending for a few miles, becomes exposed to the intense cold of the upper regions of the atmosphere. It then very soon loses all its heat, and returns from the equator much colder than it came. It is impossible that we can receive any heat directly from the equatorial regions by means of aërial currents. It is perfectly true that the south-west wind, to which we owe so much of our warmth in this country, is a continuation of the anti-trade. But the heat which this wind brings to us is not derived from the equatorial regions. This will appear evident, if we but reflect that, before the upper current descends to the snow-line after leaving the equator, it must traverse a space of at least 2000 miles; and to perform this long journey several days will be required. During all this time the air is in a region below the freezing-point; and it is perfectly obvious that by the time it begins to descend it must have acquired the temperature of the region in which it has been travelling. If such be the case, it is evident that a wind whose temperature is below 32° could never warm a country such as ours, whose temperature does not fall below 38° or 39°. The heat of our south-west winds is derived, not from the equator but from the warm water of the Atlantic-in fact, from the Gulf-stream. The upper current derives its heat after it descends to the earth. There is one way, however, whereby heat is indirectly conveyed from the equator by that current; that is, in the form of aqueous vapour. In the formation of one pound of water from aqueous vapour, as Professor Tyndall strikingly remarks, a quantity of heat is given out sufficient to melt five pounds of cast iron. It must, however, be borne in mind that the greater part of the moisture of the south-west and west winds is derived from the ocean in temperate regions. The upper current receives the greater part of its moisture after it descends to the earth. The greater part of the moisture received at the equator is condensed and falls as rain in those regions. These, as well as many other considerations which might be stated, lead to the conclusion that, in order to raise the mean temperature of the whole earth, water should be placed along the equator-and not land, as is generally believed. For if land is placed at the equator, we prevent the possibility of conveying the sun's heat from the equatorial regions by means of ocean-currents. The transference of heat could only then be effected by means of the upper currents of the trades; for the heat conveyed by conduction along the solid crust, if any, can have no sensible effect on climate. But these currents, as we have just seen, are ill adapted for conveying heat. The surface of the ground at the equator becomes intensely heated by the sun's rays. This causes it to radiate off its heat more rapidly into space than a surface of water heated under the same conditions. Again, the air in contact with the hot ground becomes also more rapidly heated than in contact with water; and consequently the ascending current of air carries off a greater amount of heat. But if the heat thus carried away were transferred by means of the upper currents to high latitudes and there employed to warm the Heat as a Mode of Motion, article 240. VOL. VI.-NO. LVIII. 11 earth, then the heat thus conveyed might to a considerable extent compensate for the absence of ocean-currents, and land at the equator might in this case be nearly as well adapted as water for raising the temperature of the whole earth. But such is not the case; for the heat carried up by the ascending current at the equator is not employed in warming the earth, but is thrown off into cold stellar space above. This ascending current, instead of being employed in warming the globe, is in reality one of the most effectual means that the earth has of getting quit of the heat received from the sun, and of thus retaining itself at a much lower temperature than it would otherwise be. It is in the equatorial regions that the earth loses as well as gains the greater part of its heat. So of all places it is here that we ought to place the substance best adapted for preventing the dissipation of the earth's heat into space if we wish to raise the general temperature of the earth. Water, of all substances in nature, seems to possess this quality to the greatest extent; and, besides, it is a fluid, and therefore adapted by means of currents to carry the heat which it receives from the sun to every corner of the globe." VI-NOTES ON CONTINENTAL GEOLOGY AND PALEONTOLOGY. REC (PART I) ECENT considerations of health having induced me to spend from five to six months on the continent, I beg to submit to the readers of the GEOLOGICAL MAGAZINE the result of my notes made during my journey, which may perhaps prove not entirely uninteresting. I. On the Cretaceous System.-All the foreign geologists with whom I have had occasion to converse in France, Switzerland, and Italy, concur in the opinion that no country has been better studied than Great Britain, and that the Museum of the Geological Survey and its published Maps are unsurpassed by any works of a similar kind hitherto produced. Our geologists have done their work well, and justly deserve the favourable judgment so liberally bestowed upon them by their continental colleagues; but we must not therefore suppose that our geological work is perfected and that we have no more to learnfor example-that our classification of British strata is either complete or entirely satisfactory. It is absolutely necessary we should know and compare the labours of continental observers with our own and see whether their discoveries or hints might not lead us to 1 Trans. of Glasgow Geol. Soc. vol. ii. part iii. p. 185; Phil. Mag. Feb. and June, 1867. 2 See "De la Science en France" by Jules Marcou, 1869. This work, to which I would call the attention of British geologists, is being published in numbers, and treats of the Imperial School of Mines, the Geological Map of France, the Academy and Institute of France, and of the Museum of Natural History in Paris, and may be obtained from C. Reinwald, Bookseller, 15, Rue des Saint Pères, Paris; or through Messrs. Trübner & Co. improve or correct our system: at the same time carefully avoiding the introduction of any foreign terminology on English ground until such may have become a well-recognized and absolute necessity. With reference to the Cretaceous system and its divisons there still remains in England ground for improvement, but I must at the same time remind the English reader that the French, Swiss, and German geologists are far from unanimous in their terminology, or as to the number or value of all their divisions. Very earnest discussions are now in active progress between those who in France are best qualified to express an opinion upon this difficult and important topic. I will not enter here upon the general literature of the subject, my object being simply to draw attention to the most recent views entertained by continental geologists. It may however be desirable for the better understanding of what will follow to mention at once that the Cretaceous system in England has been divided in the following manner: 1. Upper Chalk 4. Chloritic Marl 5. Upper Green Sand 6. Gault 7. Lower Green Sand 9. Wealden and that it is mainly due to Mr. Judd's admirable labours on the Speeton Clay, that the existence of a large portion of the Neocomien formation in England has been recently established. Mr. Judd is prepared to stand by the succession of beds which he described at Speeton,' admitting at the same time that their grouping is to a great extent arbitrary, as there are no stratigraphical breaks, and that he was guided by all he then knew of the character of the French and Jura Neocomien especially, from d'Archiac and the Swiss geologists. Mr. Judd would, however, be prepared to modify his classification so soon as sufficient cause may be shown. I am happy to be able to state also that foreign geologists have warmly acknowledged the great value of Mr. Judd's researches and inductions.-The valuable labours of Mr. C. J. A. Meyer, Prof. Morris, the Rev. T. Wiltshire, Mr. J. F. Walker, and of some others, have also materially contributed within the last few years towards clearing away some of those difficulties which still beset the final settlement of our divisions. In France the old nomenclature was rejected by M. Alcide d'Orbigny, who divided the system into the following stages: For many years M. Coquand, a very able and experienced geologist, has been devoting considerable attention to the Cretaceous 1 Quart. Journ. Geol. Soc. for August, 1868. These papers will be found in the volumes of the GEOLOGICAL MAGAZINE, the Proceedings of the Geologists' Association, and the Quarterly Journal of the Geological Society. |