There is one point to which I wish particularly to direct attention—viz., that I have studiously avoided introducing into the theories propounded anything of a hypothetical nature. There is not, so far as I am aware, from beginning to end of this volume, a single hypothetical element: nowhere have I attempted to give a hypothetical explanation. The conclusions are in every case derived either from facts or from what I believe to be admitted principles. In short, I have aimed to prove that the theory of secular changes of climate follows, as a necessary consequence, from the admitted principles of physical science.
CHAPTER II.
OCEANS-CURRENTS IN RELATION TO THE DISTRIBUTION OF HEAT OVER THE GLOBE.
The absolute Heating-power of Ocean-currents.—Volume of the Gulf-stream.—Absolute Amount of Heat conveyed by it.—Greater Portion of Moisture in inter-tropical Regions falls as Rain in those Regions.—Land along the Equator tends to lower the Temperature of the Globe.—Influence of Gulf-stream on Climate of Europe.—Temperature of Space.—Radiation of a Particle.—Professor Dove on Normal Temperature.—Temperature of Equator and Poles in the Absence of Ocean-currents.—Temperature of London, how much due to Ocean-currents.
The absolute Heating-power of Ocean-currents.—There is perhaps no physical agent concerned in the distribution of heat over the surface of the globe the influence of which has been so much underrated as that of ocean-currents. This is, no doubt, owing to the fact that although their surface-temperature, direction, and general influence have obtained considerable attention, yet little or nothing has been done towards determining the absolute amount of heat or of cold conveyed by them or the resulting absolute increase or decrease of temperature.
The modern method of determining the amount of heat-effects in absolute measure is, doubtless, destined to cast new light on all questions connected with climate, as it has done, and is still doing, in every department of physics where energy, under the form of heat, is being studied. But this method has hardly as yet been attempted in questions of meteorology; and owing to the complicated nature of the phenomena with which the meteorologist has generally to deal, its application will very often prove practically impossible. Nevertheless, it is particularly suitable to all questions relating to the direct thermal effects of currents, whatever the nature of these currents may happen to be.
In the application of the method to an ocean-current, the two most important elements required as data are the volume of the stream and its mean temperature. But although we know something of the temperature of most of the great ocean-currents, yet, with the exception of the Gulf-stream, little has been ascertained regarding their volume.
The breadth, depth, and temperature of the Gulf-stream have formed the subject of extensive and accurate observations by the United States Coast Survey. In the memoirs and charts of that survey cross-sections of the stream at various places are given, showing its breadth and depth, and also the temperature of the water from the surface to the bottom. We are thus enabled to determine with some precision the mean temperature of the stream. And knowing its mean velocity at any given section, we have likewise a means of determining the number of cubic feet of water passing through that section in a given time. But although we can obtain with tolerable accuracy the mean temperature, yet observations regarding the velocity of the water at all depths have unfortunately not been made at any particular section. Consequently we have no means of estimating as accurately as we could wish the volume of the current. Nevertheless, since we know the surface-velocity of the water at places where some of the sections were taken, we are enabled to make at least a rough estimate of the volume.
From an examination of the published sections, I came to the conclusion some years ago[11] that the total quantity of water conveyed by the stream is probably equal to that of a stream fifty miles broad and 1,000 feet deep,[12] flowing at the rate of four miles an hour, and that the mean temperature of the entire mass of moving water is not under 65° at the moment of leaving the Gulf. But to prevent the possibility of any objections being raised on the grounds that I may have over-estimated the volume of the stream, I shall take the velocity to be two miles instead of four miles an hour. We are warranted, I think, in concluding that the stream before it returns from its northern journey is on an average cooled down to at least 40°,[13] consequently it loses 25° of heat. Each cubic foot of water, therefore, in this case carries from the tropics for distribution upwards of 1,158,000 foot-pounds of heat. According to the above estimate of the size and velocity of the stream, which in [Chapter XI.] will be shown to be an under-estimate, 2,787,840,000,000 cubic feet of water are conveyed from the Gulf per hour, or 66,908,160,000,000 cubic feet daily. Consequently the total quantity of heat thus transferred per day amounts to 77,479,650,000,000,000,000 foot-pounds.
This estimate of the volume of the stream is considerably less by one-half than that given both by Captain Maury and by Sir John Herschel. Captain Maury considers the Gulf-stream equal to a stream thirty-two miles broad and 1,200 feet deep, flowing at the rate of five knots an hour.[14] This gives 6,165,700,000,000 cubic feet per hour as the quantity of water conveyed by this stream. Sir John Herschel’s estimate is still greater. He considers it equal to a stream thirty miles broad and 2,200 feet deep, flowing at the rate of four miles an hour.[15] This makes the quantity 7,359,900,000,000 cubic feet per hour. Dr. Colding, in his elaborate memoir on the Gulf-stream, estimates the volume at 5,760,000,000,000 cubic feet per hour, while Mr. Laughton’s estimate is nearly double that of mine.