Physical Conditions of Inland Seas. 547 



■nnder-current serves, by continually carrying back a portion of tbe denser Mediter- 

 ranean water, to prevent the accumulation of salt in the Mediterranean basin, which 

 would otherwise result from the continual inflow of sea water in place of thQ fresh 

 water that has passed off by evaporation. This outflow the author believes (with 

 Captain Maury) to depend upon the excess of weight of the Mediterranean column 

 above the Atlantic column ; its immediate physical cause, therefore, being exactly the 

 same as that of the outflow of bottom-water from tbe Polar area.^ 



It will be obvious from what has preceded, that tbe Gibraltar in-current will pro- 

 duce very little change in the temperature of the Mediterranean ; since, as its depth 

 on the "ridge" is little more than 100 fathoms, the water which it brings-in will 

 entirely belong to that superficial stratum whose temperature depends upon solar 

 radiation. The summer temperature of that stratum in the Atlantic is a few degrees 

 below that of the superficial stratum of the Mediterranean; and the winter tempera- 

 tures of tbe two are about the same. We may, therefore, throw the thermal in- 

 fluence of the Gibraltar in-current out of consideration, except as regards its influence 

 in slightly lowering the se^^/a^e-temperature of the westernmost portion of the 

 Mediterranean. The temperature of the deeper stratum will never be affected by it, 

 since it is always below that of the water entering by the Strait, which will float 

 upon it in virtue alike of its lower salinity and of its higher temperature. 



Tbe Summer temperature of the surface of the Meditei-ranean, where not depressed 

 by the entrance of Atlantic water, generally ranges between 70° and 80". But this 

 temperature rapidly falls in passing from the surface downwards ; as much as 20^ 

 being sometimes lost in the first 30 fathoms. In the Western basin, the thermometer 

 generally sinks at 50 fathoms to 55" or 56° ; and below this we observe very little 

 change down to 100 fathoms, at which it usually stands at 54°. From this to the 

 bottom, however deep that bottom may be, the temperature continues uniform^ the 

 water between 100 and 1,600 fathoms having absolutely the same temperature of 54-" 

 throughout. In tbe Eastern basin, of which the axis lies about 2° further to the 

 south, the heat of the superficial stratum extends somewhat further down ; but the 

 uniform temperature is always reached at less than 200 fathoms; and from that 

 depth to the bottom at (it may be) 2,000 fathoms, the temperature of 56° degrees is 

 found everywhere to prevail. 



Now, it is obvious from these facts (1) that Depth per se has no effect in reducing 

 temperature ; (2) that the uniform temperature of the Mediterranean from 100 or 

 200 fathoms to the bottom must be determined by some local condition ; whilst (3) as 

 this condition might be expected to have tbe same eflfect on the water of the outside 

 Atlantic under the same parallel of latitude, the coldness of its lower stratum, 

 ranging, from 900 fathoms downwards, between 39° and. 36'5°, must be diie to the 

 importation of water from a Polar source. What, then, is the determining condition 

 of the uniform temperature of 64° in the Western basin of the Mediterranean, and 

 of 56° in tbe Eastern 2 



In the Eeport (for 1870) of his fi^rst researches in the Mediterranean, the Author 

 attributed it to tbe swijacent infiuence of the warm crust of the earth, tbe tempera- 

 ture of wliich in the Mediterranean area^ as indicated by the uniform temperature 

 of a deep cave in the Island of Pantellaria, and by that of the deepest tanks in Malta, 

 seems to be 54°. But he is now inclined to believe that this coincidence is accidental 

 only, and that, no coLler water being admitted from without, the uniform tempera- 

 ture of the mass of Mediterranean water in each basin really corresponds to its lowest 

 winter mean, or isocheimal temperature. According to the best information he has 

 been able to obtain, th€ Winter temperature of the Western basin is 54* from 

 the siirface to the bottom, while that of the Eastern basin is 56' throughout; the 

 slight excess in the latter being due to the small difi"erence in latitude, with the effect 

 of the hot winds from Central Africa. As the sun gains power, its radiation raises 

 the temperature of tbe superficial stratum, but doe& not penetrate far downwards ; 

 and thus the great mass of the water beneath remains unaffected by it. If, on the 

 other hand, tbe Winter temperature of the surface were to be reduced, that reduction 

 would affect the entire mass ; because, as the surface-water is cooled, it avaks, and 

 difi'uses its cold through the water beneath. 



Thus the bottom-temperature of a deep Inland Sea may be expected to depend 



1 So in the case of the Black Sea, it was maintained by the Author that the e.xcess of Specific 

 Gravity of the M'^ea.n water must produce an inward under-current throujjh the Dardanelles and 

 the Bosphorus ; and this prediction, which was affirmed by Captain Spratt to have been disproved 

 by his own previous researches, has recently been verified by the " Shearwater " investigations. 



