--

There she sate now, in a sort of stupor -a sort of dreary bewilderment - the illusion of the Pythian gone desire of dream and of prophecy alike extinct pressing her hands together, and muttering to herself, "What has happened?what have I done?"

effect of pressure on his thermometers), and stamped with the great weight of his authority by Sir John Herschel. That it is theoretically true, all Physicists must admit. Wherever, in the water of a large basin, different Temperatures prevail, there must be different densities, causing Three hours later you would not have differences of lateral and downward presrecognized the same face that you see sure; and equilibrium can only be renow. For then the bravery, the honour, stored by such interchange between difthe loyalty of the girl's nature had assert- ferent parts of the liquid mass, as will ed their command. Her promise had equalize its temperature throughout. been given to one man -it could not be But if such a difference between the recalled. Thought itself of any other Temperatures of the two ends of the man must be banished. On her hearth basin be constantly and persistently lay ashes and tinder- the last remains maintained, a constant circulation will be of every treasured note from Graham kept up; the heavier, because colder, Vane; of the hoarded newspaper extracts water persistently gravitating to the that contained his name; of the dry trea- bottom, and flowing along the floor of tise he had published, and which had the basin towards the warmer end; whilst made the lovely romance-writer first de- the lighter, because warmer, water will sire to know something about politics." rise to the surface and flow towards the Ay, if the treatise had been upon fox- colder end, where, by being again reduced hunting, she would have desired to know in temperature, it will sink to the botsomething about that! Above all, yet tom, and go the same round. And that distinguishable from the rest -as the this vertical Circulation has an actual sparks still upon stem and leaf here and existence in the great Ocean-basins, I there faintly glowed and twinkled — the endeavoured to show from the Temperawithered flowers which recorded that ture-phenomena collected during the Porhappy hour in the arbour, and the walks cupine Expeditions of 1869 and 1870; of the forsaken garden - the hour in and especially from the contrast between which she had so blissfully pledged her- the Thermal condition of the Mediterraself to renounce that career in art where-nean and that of the outside Atlantic unin fame would have been secured, but which would not have united Fame with Love in dreams evermore over now.

From The Contemporary Review.

der the same parallels. Although these conclusions have been disputed by several persons who consider themselves as authorities on the subject of Ocean-currents, yet as they have been accepted by such eminent Physicists as Sir John Herschel (in a letter which he was good

ON THE PHYSICAL CONDITIONS OF IN- enough to write to me shortly before his

LAND SEAS.

IN a former number of this Review

death), Sir George Airy, and Sir William Thomson (who has authorized me to express his entire agreement with me on the whole of this question), I venture to think that they may be regarded as worthy of provisional adoption.

(February, 1871), I gave a general account of that part of the inquiries in which I had been engaged during the three preceding years, which bore on the The researches of the Challenger, so subject of Ocean Circulation; and ex- far as they have gone, have fully conplained the definite Physical Theory of a firmed them; the basin of the Atlantic vertical Circulation, sustained by Tem-between the Azores and St. Thomas, perature alone, to which those inquiries from a depth of 1000 fathoms to a bottom had led. I afterwards found that a sim- lying in some parts at a depth of 2700 ilar doctrine had been previously ad- fathoms,* being occupied by water of vanced by Pouillet, as best expressing which the temperature ranges downwards the facts then known; although subse- from 40° to 34°5. That it has not been quently put aside by the general acceptance of the erroneous doctrine of a uniThe enormous depth of 3875 fathoms has been lateform Deep-sea Temperature of 39°, firstly reached, not far north of St. Thomas's. But the promulgated by Sir James Ross (on the bottom-temperature could not be obtained; for the basis of observations which we now know thermometers which had been tested to a pressure of 31-4 tons on the square inch broke at a pressure of 4 to have been rendered erroneous by the 1-2 tons. (See "Nature,” June 5.)

found to fall still lower, I believe to be due to the very limited communication which the Arctic basin has with the North Atlantic; and it seems not improbable that a considerable part of this enormous mass of almost glacial water has come all the way from the Antarctic basin. If the doctrine I advocate be correct, the bottomtemperature of the Southern Oceans will be lower than that of the Northern, on account of their free and direct communication with the Antarctic area; and the difference will be especially marked in the Pacific, since, as no Arctic water can come into it through Behring's Straits (whose depth of only 20 fathoms is occupied by a warm current passing northwards), any reduction which may be found in the temperature of its lower stratum must be mainly due to an under-flow of water all the way from the Antarctic area. It may be confidently hoped that the Challenger, whose voyage has been so planned as to enable the requisite observations to be made in all these Oceanic areas, may succeed in collecting a body of facts which will either demonstrate the correctness of this theory, or will furnish materials for a better one.

and scarcely any water comes from the land that encloses it. Thus, then, the strong and constant current which streams into it through the Strait of Babel Mandeb is fully accounted for. In the Baltic, on the other hand, the loss by evaporation is far smaller than the return by rain and rivers; so that its level would be raised, and its area increased, were it not for the outflow of the excess which takes place through the Baltic Sound and the Great and Little Belt. And the same is the case with the Black Sea, the overflow of which is carried off by the out-current which sets through the Bosphorus, the Sea of Marmora, and the Dardanelles, into the Egean.

But in each of these cases, there is an inequality between the density of the water within the Strait, and that of the water outside. In the case of the Red Sea, the almost uncompensated evaporation tends to increase the salinity, and therefore the specific gravity of its water; and there is consequently an excess of deep lateral pressure on the inside of the Strait of Babel Mandeb, which will necessarily produce an under-current towards the outside. The existence of this under-current has not yet been practically demonstrated; but, as Captain Maury pointed out, it may be fairly assumed; since, as the place of the vast quantity of fresh water always passing off by evaporation is taken by an influx of salt water, the proportion of salt in the basin of the Red Sea would be undergoing a constant increase, if it were not thus kept down. Of such an increase there is no evidence whatever, the excess of salt in the water being scarcely greater than in that of the Mediterranean; whilst there is no reason to believe that any such deposits of salt are going on upon its shores or bottom, as will be presently shown to be forming around the Caspian. On the other hand, the water of the Baltic and of the Black Sea is reduced in salinity by the excessive influx of river-water; so that the former has only about one-fifth, and the latter less than one-half, of the density of Ocean-water. Hence the greater lateral pressure in the Straits by which the former communicates with the North Sea and the latter with the Medi

But, as I pointed out in my former paper, the same Physical Theory applies to the double currents, which are known to pass through straits connecting Inland Seas with the Ocean or with each other. Every Inland Sea is subject to two agencies tending to alter its level; namely, evaporation from its surface, by which its level will be reduced; and a return of water by rain and river, by which its level will be raised. Now, it is almost a physical impossibility that these two agencies should exactly balance one another, except in the cases of seas entirely shut in; in which they come to a balance by the alteration of the level, and the consequent extension or contraction of the area. This is well known to be the condition of the Dead Sea, the area of which has been reduced by excess of evaporation, until its loss of water is no longer greater than the amount returned by the Jordan and other streams that discharge themselves into it; and the same will be presently shown to be the case with the Caspian. In the Red Sea, an enormous evaporation-annually amount-terranean, is from the outside; and an ing at the very lowest estimate to a stratum of eight feet, and by some estimated at twenty-three feet per annum,· is constantly going on, uncompensated by return either from rain or rivers; for the area of the Red Sea is nearly rainless,

inward under-current would be thus produced, which, by carrying salt water into the basin, would prevent its salinity from being further reduced. The existence of this under-current, as I showed in my former paper, has long been known in

the case of the Baltic; whilst in the case of the Black Sea, "it might be safely predicted on the double ground of à priori and à posteriori necessity." The truth of that prediction (as will presently appear) has already been signally verified.

grapher to the Admiralty to cause a series of experiments to be there made with the "current-drag" which had been successfully used in the experiments made by Captain Nares and myself in the Strait of

Gibraltar.

Being anxious to obtain more complete and conclusive evidence as to the with great skill by Captain Wharton, These experiments were carried out outward under-current in the Strait of who succeeded Captain Nares in comGibraltar, than it had been possible for mand of the Shearwater; and the summe to obtain in my visit to the Med-mary of their results which I shall now iterranean in the Porcupine, I gladly give, is derived from the official account availed myself of the opportunity kind- furnished by him to the Admiralty, of ly offered me in the following year by which I have received a. copy by the the Hydrographer to the Admiralty, to kindness of the Hydrographer. go out with Captain Nares (now in command of the Challenger Expedition) that the Dardanelles and Bosphorus surAlthough it is commonly supposed in the Shearwater, then on her way to face-currents are overflow-currents, carrycomplete the survey of the Gulf of Suez, ing off the excess of fresh water disand to co-operate with him in a further charged by rivers into the Black Sea, yet series of inquiries. The result of those it is now clear that they are in great inquiries was to show that both the up- measure wind-currents. During about per in-current and the under out-current three quarters of the year, the wind blows are much more influenced by tidal action pretty steadily from the N. E., that is, than had been previously supposed: both down the Straits; and, as a rule, the of them undergoing a regular reversal stronger and more continuous the wind, every six hours upon the "ridge" at the the stronger is the surface out-current. broad western embouchure of the Strait; On calm days, the out-current of the Darwhile, at the Gibraltar end of the Strait, danelles is usually slack; and if, as somethe in-current ordinarily comes to stand, if it be not actually reversed, when the S.W., its flow may be entirely checked. a times happens, a strong wind blows from antagonized by the west-flowing tide- It requires a continuance of strong S. W. wave which adds strength to the out-cur- wind, however, to reverse its direction; rent beneath. Thus the quantity of water and its rate, when thus reversed, is never which flows in each direction is by no equal to that of the out-current. The means as great as might be supposed speed of the Dardanelles current varies from observations made at the period of at different parts of the Strait, according its most rapid movement. The balance to its breadth, — being usually about one of the upper current is most decidedly knot per hour at Gallipoli, and three inwards; that of the under-current is knots in the less considerably, though still decidedly Kaleksi, where, with a strong N. E. wind "Narrows" at Chanak outwards, -as was shown not merely by it is sometimes as much as four and athe results of our Current-drag experi- half knots; the average of the whole bements, but by the fact clearly indicated ing estimated by Captain Wharton at one by the Hydrometer, that Mediterranean and a-half knots. The Bosphorus curwater flows down the Atlantic slope of rent has not been so carefully studied as the marine watershed, and is traceable that of the Dardanelles; but Captain on the bottom to a considerable distance. | Wharton states that its rate is greater, That this under-current has by no means averaging about two and a-half knots per the force or proportionate volume of that hour, apparently in consequence of the which has since been shown to exist in limitation of its channel, which is scarcethe Dardanelles, is simply due to the ly wider at any point than is the Dardafact, that the difference in specific gravi- nelles at the " Narrows." It continues ty between the water of the Mediter- to run, though at a reduced rate, when ranean and that of the Atlantic is very there is no wind and it is only in winter, small in comparison with that which ex- after a continued S. W. gale of long duists between the water of the Black Sea ration, that a reversal of the Bosphorus and that of the Ægean. current ever takes place.

As I understood that the Shearwater, when her survey of the Gulf of Suez had been completed, would proceed to the Dardanelles, I requested the Hydro

LIVING AGE.

VOL. III.

144

:

It might have been supposed that, as the greatest depth of these two Straits does not exceed fifty fathoms, the determination of the question as to the exist

ence of an under-current would be a comparatively easy matter. But it is rendered It was a wonderful sight to see this series difficult by the very rapidity of the movement, alike in the upper and in the lower

as the bottom-pressures of the two are equal, their lateral pressures will also be equal, and there will be no under-current so long as this condition lasts. But so soon as, on the cessation of the S.W. wind, the level of the Black Sea is lowered by a surface-outflow, the Egean column comes to be the heavier, and its excess of lateral pressure produces a deep inflow. And when this outflow is further aided by a N.E. wind, so that the levels of the two seas are equalized, or there is even an excess of elevation at the Ægean end, the greater weight of the Egean column will produce a greater lateral pressure, and will consequently increase the force of the inward under-current.

when restricted to a narrow channel; and the "creeping-flow" (I have never designated it as a "current") of Polar water along the Ocean bottom, which brings a glacial temperature into the Intertropical zone, is thus found to have an adequate vera causa, in the excess of deep lateral pressure exerted by the Polar column whose density has been augmented by cold, over that of the Equatorial column whose density has been diminished by heat.

Professor Huxley, however, while fully accepting these general propositions, and laying special stress on the contrast between the Temperature-phenomena of The two following cases are peculiarly the Mediterranean and those of the outillustrative of the effects of differences in side Ocean, as evidence of the General downward pressure in the production of Circulation for which I contend, has reunder-currents. The exit of the water cently expressed the opinion in the pages brought down by the Hudson river is so of this Review (Vol. xxi., p. 840), that much impeded by the "Narrows" of the cause of the surface in-current New York Harbour, that the surface-lev- through the Strait of Gibraltar, which is el of the river is always higher than that constantly bringing into the basin of the of the sea outside; and as the difference Mediterranean a vast body of Atlantic is ordinarily sufficient to do more than water, has not been shown to lie in that compensate for the excess in the weight excess of evaporation from the surface of of the column of sea-water outside, above the Mediterranean above the return by that of the column of river-water inside, rain and rivers, to which, since the first no deep inflow of sea-water takes place. promulgation of this doctrine by Dr. But during the dry summer-season, the Halley, it has been usually attributed. level of the river comes down so nearly cannot but think that if my friend had to an equality with that of the sea, that looked a little more carefully into the evithe outside column becomes the heavier; dence on this point, he would have and a deep inflow of salt water then scarcely used his authority to call in takes place, extending good way up the question a doctrine, which may, I think, river, though the surface outflow consist- be considered as being as well estabing of water thus rendered brackish, con-lished as any doctrine in Physical Geogtinues for nine out of the twelve tidal raphy.

I

hours. Again it was pointed out by Sir In the first place prima facie evidence William Thomson at the Edinburgh in its favour is afforded by the conMeeting of the British Association, that stantly-maintained excess in the salinity the persistence of a surface current up a of Mediterranean water above that of the loch that opens from the sea, when a outside Atlantic. This excess is greater wind continuously setting inwards had than Prof. Huxley has stated; for the raised a "head of water at its farther specific gravity of the surface-water of extremity, can only be accounted for by the Mediterranean, where subjected to a compensating outward undercurrrent; great evaporation, and not reduced by the which will be maintained by the excess inflow from the Atlantic, ranges as high of pressure at the head of the loch, so as 10294, and the bottom-water to 10302, long as the level of the water is there while that of Atlantic water averages kept up by the persistence of the in- 10265. And the excess of the saline ward drift-current. constituents, as determined by chemical The fact may now, therefore, be con-analysis, ranges as high as nine per cent. sidered as put beyond question, that a That there is no increase in the proporslight excess of downward pressure, tion of salt, notwithstanding the enorwhether arising from difference of specific mous amount daily brought by the Gibgravity, or from difference of level, is quite adequate to produce movement in great bodies of water, which movement may have the rate and force of a current

raltar current into the Mediterranean basin, is simply due to the fact that the outward under-current of dense Mediterranean water is constantly returning to