Page:Encyclopædia Britannica, Ninth Edition, v. 3.djvu/816

798 not allow it to give immediate passage to the overflow, a portion of the current continues to pass onwards along the southern shore, ami, when more than usually strong, even completes the entire circuit of the sea. There are no per ceptible tides in this basin. As might be expected from the foregoing, the salinity of Black Sea water varies (like that of the Baltic) at dif ferent periods of the year ; but in consequence of the much greater total mass of water contained in the deeper part of this basin, the variation of its salinity is by no means so great as that of Baltic waters, the usual range of its sp. gr. being from about 1 - 012 to 1 014, which corresponds to a little less or a little more than half the salinity of ordinary sea-water. The most contradictory notions have prevailed as to the influence of the Euxine waters on those of the Mediter ranean, some writers having represented the rivers of the former as important contributors to the maintenance of the level of the Mediterranean, which the enormous evaporation from tbat area is always tending to reduce ; whilst others assert that the Bosphorus and Dardanelles currents are entirely due to the agency of wind. A valuable datum is afforded by the condition of the Caspian, in the closed basin of which, contracted by a reduction of its level to 80 feet below that of the Black Sea, an equality is now established between the amount of water lost by evaporation and that which is restored by its rivers and by the rainfall on its own surface. The areas of the Caspian and of the Euxiue are not very different; and though the axis of the former basin lies north and south, while that of the latter lies east and west, so that the northern portion of the Caspian is colder, and the southern portion warmer, than the northern and southern portions of the Black Sea, the annual average temperatures, and consequently the total evaporation, of the two areas cannot differ much. Now, the drainage area of the Volga is equal to that of the Danube, the Dnieper, and the Dniester taken together ; the Ural, with the two Transcaucasian rivers, Kur and Araxes, may be considered as equalling the Don ; and thus the Bug and the rivers of the Caucasus and Asia Minor may be regarded as furnishing the excess of water discharged into the Black Sea above that which is received by the Caspian. Hence, as the whole of the river and rain water annually discharged into the basin of the Caspian is only sufficient to replace that which is lost by evaporation during the same period, it follows that we may in like manner regard the principal rivers of the Black Sea as only fulfilling the same function ; consequently, if the Bos phorus were closed, the water which they pour into the Euxine basin would not produce any elevation of its level, being entirely dissipated by evaporation. Thus the water which the Black Sea has to spare for the Mediterranean only represents the excess of its river supply above the total river supply of the Caspian ; and that this excess is small in amount appears from the fact that the salinity of the water of the ^Bgean is not sensibly reduced by it below that of the Mediterranean. But that there is some excess is evident from the consideration that if the evaporation of the Black Sea were merely neutralized by the return of fresh water, its water would have the salinity of that of the great basin with which it is in free communication, instead of containing only about half its proportion. It is further evident, on the other hand, that a continual efflux of the half-salt water of the Black Sea, to be replaced only by the fresh water discharged into its basin by rain and rivers, would in time completely drain that basin of its salt ; and as its proportion, though liable to seasonal variation, undergoes no sensible diminution from year to year, it is obvious that the salt which passes out must be replaced by a re-eutry of ^Egoan water. The mode in which this re placement is effected has teen recently elucidated by a careful examination of the currents of the Black Sea straits, of which an account will be presently given. It is during the winter months, when a large proportion of the drainage area of the Black Sea rivers is covered with snow, that the supply of water is at its minimum ; but it is then that the evaporation from its surface is also at its minimum; so that there is no reason to suppose that the level of the Black Sea ever falls below that of the ^Egean. There can be no reasonable doubt that during the spring and early summer, when the melting of the snows causes the rivers to swell to their highest, the quantity of fresh water thus brought into the basin, being greater than that which is lost by evaporation (as is shown by the general reduction which then takes place in the salinity of its con tents), would cause a considerable rise of level, if this were not kept down by the outflow through the straits.

Dardanelles and Bosphorus Currents.—It has been known from very early times that a current, usually of considerable strength, sets outwards through the Black Sea straits during a large part of the year, its rate being subject, however, to considerable variation in accordance with the breadth of the channel, and also with the force and direction of the wind. Thus, when the N.E. wind is of average strength, the rate of the current at Callipoli is about 1 knot per hour; whilst in the &quot;Narrows&quot; at Chanak Kaleski it is about 3 knots, increasing with a strong wind to about 4?&amp;gt; knots. In calm weather the out-current of the Dardanelles is usually slack; and if, as sometimes happens even during the general prevalence of N.E. winds, the wind should suddenly blow strongly from the S.W., the surface outflow may be entirely checked. It requires a continuance of strong S.W. wind, however, to reverse its direction; and its rate, when thus reversed, never equals that of the out-current. The Bosphorus current has not been as carefully studied as that of the Dardanelles ; but its rate is greater, in accordance with the limitation of its channel, which is scarcely wider at any point than the &quot;Narrows&quot; of the Dardanelles. It con tinues to run, though at a reduced rate, when there is no wind, and is not known to be ever reversed except in winter after a S.W. gale of long duration. Even then it appears that the reversal is confined to the superficial stratum, the direction of the sub-surface water-weeds proving that there is still an outflow from the Black Sea into the ^Egean. Hence it cannot be reasonably maintained that it is by this occasional and superficial reversal that the immense mass of salt continually being carried outwards by the Bosphorus and Dardanelles currents is restored to the Black Sea basin. The existence of an inward under-current (although controverted by an authority of weight) has been clearly demonstrated by the recent experimental researches of Captain Wharton, E.N., of H.M. surveying ship &quot;Shear water.&quot; By the use of a &quot; current-drag,&quot; .so constructed and suspended as always to present a large vertical surface, it was found that when the outward surf ace- current was at its strongest there was an inward under-current suffi ciently strong and rapid to carry inwards the suspending buoy. The difference in specific gravity of water obtained from different depths was found, in Captain Wharton s investi gations (as in those previously made by Dr Carpenter, in conjunction with Captains Calver and Narcs, in the Strait of Gibraltar), to afford, under ordinary circumstances, a very sure indication of the direction of the movement of each stratum, the heavy water of the ^Egean, as a rule, flowing inwards, and the lighter water of the Black Sea flowing outwards. And it was indicated alike by both modes of inquiry that the two strata move in opposite directions, one over the other, with very little intermixture 