164i REPORT — 1873. 



surface-current. And thus the whole contribution of the great rivers th.at discharge 

 themselves into the Black Sea, to the maintenance of the level of the Mediterra- 

 jiean, is represented by an outflow through the Dardanelles by no means exceed- 

 ing the amount brought down by a single considerable river. 



AVe now turn to the Mediterranean, and shall again use the Caspian as a basis 

 on which we may form some kind of approximative estimate as to the proportion 

 between the evaporation from its surface and the return by river-flow. 



In the first place, the area of the Mediterranean, including the JEgQ&n and the 

 Adriatic, is between four and Jive times the present area of the Caspian ; so that, 

 taking the evaporation over equal areas of the two seas to be the same, the quan- 

 tity of return that would be needed to keep up the level of the Mediterranean 

 would be between four and five times as great as that which suffices to maintain 

 that of the Caspian. But looking to the fact that the principal part of the area 

 of the Mediterranean lies east and west between the parallels of 32° and 40° N. lat., 

 whilst that of the Caspian lies north and south between the parallels of 36° and 

 46°, it seems obvious that this difference alone would cause the evaporation of the 

 Mediterranean to be much greater for equal areas than that of the Caspian. The 

 ordinary summer temperature of a considerable part of the eastern basin of the 

 Mediterranean is not much below 80° : Dr. Carpenter has himself seen it ranging 

 from 75° to 80° between Malta and jVlexandria in the early part of October. And, 

 notwithstanding the curious northern bend by which the summer isotherm of 80° 

 is carried through Greece"and Asia Minor, along tlie southern shore of the Black 

 Sea, it only just touches the southern basin of the Caspian, the summer tempera- 

 ture of nearly the whole of this sea being below that of the northernmost parts of 

 the Mediterranean. The difference is far greater, however, during the winter 

 months. Taking the lowest winter temperature of the Mediterranean at Prof. 

 Huxley's aver.ige of 48° (and Dr. Carpenter has reason to believe that this is some 

 degrees too low for the eastern basin, whilst it is not at all too high for the 

 western), we find the January mean of the Caspian to range from 40° at its 

 southern extremity to 30° in its middle basin, while its] northern b.asin is crossed 

 by the January isotherm of 20°. Hence, as regards temperature alone, the mean 

 annual excess is largely on the side of the Mediterranean. But there is another 

 element not less important — the extreme dryness of the hot winds which blow over 

 the Mediterranean ( especially its eastern basin) from the great African deserts, and 

 which take up an enormous amount of moisture in their course. 



We should not be far wrong, then, in assuming that, to counteract this enormous 

 evaporation, the volume of river- water poured into the Mediterranean ought to be 

 at least six times that received by the Caspian. But what is the actual amount of 

 that supply ? Along tlie whole Africancoast, from the Strait of Gibraltar to the 

 Nile, there is nothing that can be called a large river. Around the whole Levant 

 there is the same deficiency. And thus, with the exception of the Nile and of the 

 Po (a slow-flowing river of very moderate volume), no great body of water is 

 poured into the eastern basin of the Mediterranean, save the overflviv of the Black 

 bea, which comes down through the Bosphorus and Dardanelles. How small a 

 contribution is made by this overflow to the maintenance of the general level of 

 the MediteiTanean, seems apparent from the fact that the specific gravity of the 

 water of the ^gean, with which it first mingles, is scarcely, if at all, lowered by 

 the intermixture of the half-salt stream which discharges itself into the part of it 

 most remote from its communication with that larger basin. Into the western 

 basin of the Mediterranean no other considerable rivers discharge themselves than 

 the Rhone and the Ebro. Thus the sum total of the supply brought into the 

 whole Mediterranean area by great rivers may be expressed by the Nile, one half 

 of the Dardanelles surface-current, the Po, the Rhone, and the Ebro. And if we 

 add to these the " ten submarine springs of fresh water which are known to burst 

 up in the Mediterranean," it seems perfectly clear that we cannot make that total 

 any thing like six times the amount which is brought into the Caspian by the 

 Volga, the Ural, and the Transcaucasian rivers, and which has been shown to be 

 cntireJy dissipated hy evajmration. It has been estimated by two French officers, 

 MM. Regy and Vigan*, who have recently compared the probable evaporation of 

 * Annales des Ponts et Chau8s6es, 1863 and 1866. 



