"Thus two antagonistic forces are unmasked, and, being unmasked, we discover in them a most exquisite adjustment-a compensation-by which the dynamical forces that reside in the sunbeam and the trade-wind are made to counterbalance each other, by which the climates of inter-tropical seas are regulated, and by which the set, force, and volume of oceanic currents are measured" (§§ 437 and 438, eleventh edition).

CHAPTER VII.

EXAMINATION OF THE GRAVITATION THEORY OF OCEANIC CIRCU

LATION.—LIEUT. MAURY'S THEORY (continued).

Methods of determining the Question.-The Force resulting from Difference of Specific Gravity.-Sir John Herschel's Estimate of the Force. Maximum Density of Sea-Water-Rate of Decrease of Temperature of Ocean at Equator.-The actual Amount of Force resulting from Difference of Specific Gravity.-M. Dubuat's Experiments.

How the Question may be Determined.-Whether the circulation of the ocean is due to difference in specific gravity or not may be determined in three ways: viz. (1) by direct experiment; (2) by ascertaining the absolute amount of force acting on the water to produce motion, in virtue of difference of specific gravity, and thereafter comparing it with the force which has been shown by experiment to be necessary to the production of sensible motion; or (3) by determining the greatest possible amount of work which gravity can perform on the waters in virtue of difference of specific gravity, and then ascertaining if the work of gravity does or does not equal the work of the resistances in the required motion. But Maury has not adopted either of these methods.

The Force resulting from Difference of Specific Gravity.—I shall consider first whether the force resulting from difference of specific gravity be sufficient to account for the motion of ocean

currents.

The inadequacy of this cause has been so clearly shown by Sir John Herschel, that one might expect that little else would be required than simply to quote his words on the subject, which are as follows:

"First, then, if there were no atmosphere, there would be no

Gulf-stream, or any other considerable ocean-current (as distin guished from a mere surface-drift) whatever. By the action of the sun's rays, the surface of the ocean becomes most heated, and the heated water will, therefore, neither directly tend to ascend (which it could not do without leaving the sea) nor to descend, which it cannot do, being rendered buoyant, nor to move laterally, no lateral impulse being given, and which it could only do by reason of a general declivity of surface, the dilated portion occupying a higher level. Let us see what this declivity would amount to. The equatorial surface-water has a temperature of 84°. At 7,200 feet deep the temperature is 39°, the level of which temperature rises to the surface in latitude 56°. Taking the dilatability of sea-water to be the same as that of fresh, a uniformly progressive increase of temperature, from 39° to 84° Fahr., would dilate a column of 7,200 feet by 10 feet, to which height, therefore, above the spheroid of equilibrium (or above the sea-level in lat. 56°), the equatorial surface is actually raised by dilatation. An arc of 56° on the earth's surface measures 3,360 geographical miles; so that we have a slope of 1-28th of an inch per geographical mile, or 1-32nd of an inch per statute mile for the water so raised to run down. As the accelerating force corresponding to such a slope (of 1-10th of a second, 0′′·1) is less than one two-millionth part of gravity, we may dismiss this as a cause capable of creating only a very trifling surfacedrift, and not worth considering, even were it in the proper direction to form, by concentration, a current from east to west, which it would not be, but the very reverse.'

It is singular how any one, even though he regarded this conclusion as but a rough approximation to the truth, could entertain the idea that ocean-currents can be the result of difference in specific gravity. There are one or two reasons, however, which may be given for the above not having been generally received as conclusive. Herschel's calculations refer to the difference of gravity resulting from difference of temperature; but this is only one of the causes to which Maury * "Physical Geography," article 57.

appeals, and even not the one to which he most frequently refers. He insists so strongly on the effects of difference of saltness, that many might think that, although Herschel may have shown that difference in specific gravity arising from difference of temperature could not account for the motion of ocean-currents, yet nevertheless that this, combined with the effects resulting from difference in saltness, might be a sufficient explanation of the phenomena. Such, of course, would not be the case with those who perceived the contradictory nature of Maury's two causes; but probably many read the "Physical Geography of the Sea" without being aware that the one cause is destructive of the other. Again, a few plausible objections, which have never received due consideration, have been strongly urged by Maury and others against the theory that oceancurrents can be caused by the impulses of the winds; and probably these objections appear to militate as strongly against his theory as Herschel's arguments against Maury's.

There is one trifling objection to Herschel's result: he takes 39° as the temperature of maximum density. This, however, as we shall see, does not materially affect his conclusions.

Observations on the temperature of the maximum density of sea-water have been made by Erman, Despretz, Rossetti, Neumann, Marcet, Hubbard, Horner, and others. No two of them have arrived at exactly the same conclusion. This probably arises from the fact that the temperature of maximum density depends upon the amount of salt held in solution. No two seas, unless they are equal as to saltness, have the same temperature of maximum density. The following Table of Despretz will show how rapidly the temperature of both the freezing-point and of maximum density is lowered by additional amounts of salt:

He found the temperature of maximum density of sea-water, whose density at 20°C. was 1.0273, to be -3° 67 C. (25°-4 F.), and the temperature of freezing-point-2°.55 (27°-4F.).* Somewhere between 25° and 26° F. may therefore be regarded as the temperature of maximum density of sea-water of average saltness. We have no reason to believe that the ocean, from the surface to the bottom, even at the poles, is at 27°-4 F., the freezing-point.

seas.

The actual slope resulting from difference of specific gravity, as we shall presently see, does not amount to 10 feet. Herschel's estimate was, however, made on insufficient data, both as to the rate of expansion of sea-water and that at which the temperature of the ocean at the equator decreases from the surface downwards. We are happily now in the possession of data for determining with tolerable accuracy the amount of slope due to difference of temperature between the equatorial and polar The rate of expansion of sea-water from 0°C. to 100°C. has been experimentally determined by Professor Muncke, of Heidelberg. The valuable reports of Captain Nares, of H.M.S. Challenger, lately published by the Admiralty, give the rate at which the temperature of the Atlantic at the equator decreases from the surface downwards. These observations show clearly that the super-heating effect of the sun's rays does not extend to any great depth. They also prove that at the equator the temperature decreases as the depth increases so rapidly that at 60 fathoms from the surface the temperature is 62°-4, the same as at Madeira at the same depth; while at the depth of 150 fathoms it is only 51°, about the same as that in the Bay of Biscay (Reports, p. 11). Here at the very outset we have broad and important facts hostile to the theory of a flow of water resulting from difference of temperature between the ocean in equatorial and temperate and polar regions.

Through the kindness of Staff-Captain Evans, Hydrographer Philosophical Magazine, vol. xii. p. 1 (1838).

See

"Mémoires par divers Savans," tom. i., p. 318, St. Petersburgh, 1831. also twelfth number of Meteorological Papers, published by the Board of Trade, 1865, p. 16.