878 REPORT—1890. 
upwards of a million square miles, and the volume of the rivers which are dis- 
charged into them at 226 cubic miles. All this and much more is evaporated 
annually. There are two constant currents passing through the Straits of Gibraltar, 
superimposed on each other; the upper and most copious one flows in from the 
Atlantic at a rate of nearly 3 miles an hour, or 140,000 cubic métres per second, 
and supplies the difference between the rainfall and evaporation, while the under- 
current of warmer water, which has undergone concentration by evaporation, is 
continually flowing out at about half the above rate of movement, getting rid of 
the excess of salinity ; even thus, however, leaving the Mediterranean salter than 
any other part of the ocean except the Red Sea. 
A similar phenomenon occurs at the eastern end, where the fresher water of 
the Black Sea flows as a surface current through the Dardanelles, and the salter 
water of the Mediterranean pours in below it. 
The general temperature of the Mediterranean from a depth of 50 fathoms 
down to the bottom is almost constantly 56°, whatever may be its surface eleva- 
tion. ‘This is a great contrast to that of the Atlantic, which ata similar depth is 
at least 3° colder, and which at 1,000 fathoms sinks to 40°. 
This fact was of the greatest utility to Dr. Carpenter in connection with his 
investigations regarding currents through the Straits, enabling him to distinguish 
with precision between Atlantic and Mediterranean water. 
For all practical purposes the Mediterranean may be accepted as being, what it 
is popularly supposed to be, a tideless sea, but it is not so in reality. In many places 
there is a distinct rise and fall, though this is more frequently due to winds and 
currents than to lunar attraction. At Venice there is arise of from one to two 
feet in spring tides, according to the prevalence of winds up or down the Adriatic, 
but in that sea itself the tides are so weak that they can hardly be recognised, ex- 
cept during the prevalence of the Bora, our old friend Boreas, which generally 
raises a surcharge along the coast of Italy. In many straits and narrow arms of 
the sea there is a periodical flux and reflux, but the only place where tidal influence, 
properly so called, is unmistakably observed is in the Lesser Syrtis, or Gulf of 
Gabés ; there the tide runs at the rate of 2 or 3 knots an hour, and the rise and fall 
varies from 3 to 8 feet. It is most marked and regular at Djerba, the Homeric 
island of the Lotophagi; one must be careful in landing there in a boat, so as not 
to be left high and dry a mile or two from the shore. Perhaps the companions of 
Ulysses were caught by the receding tide, and it was not only a banquet of dates, 
the ‘honey-sweet fruit of the Lotus,’ or the potent wine which is made from it, 
which made them ‘ forgetful of their homeward way.’ 
The Gulf of Gabés naturally calls to mind the proposals which were made a 
few years ago for inundating the Sahara, and so restoring to the Atlantic region 
the insular condition which it is alleged to have had in prehistoric times. I will 
not allude to the English project for introducing the waters of the Atlantic from 
the west coast of Africa; that does not belong to my subject. The French scheme 
advocated by Commandant Roudaire, and supported by M. de Lesseps, was quite 
as visionary and impracticable. 
To the south of Algeria and Tunis there exists a great depression. stretching 
westward from the Gulf of Gabés to a distance of about 235 miles, in which are 
several chotts or salt lakes, sometimes only marshes, and in many places covered 
with a saline crust strong enough to bear the passage of camels. Commandant 
Roudaire proposed to cut through the isthmuses which separated the various chotts, 
and so prepare their basins to receive the waters of the Mediterranean. This done, 
he intended to introduce the sea by a canal, which should have a depth of one 
métre below low-water level. 
This scheme was based on the assumption that the basin of the chotts had been 
an inland sea within historic times; that, little by little, owing to the difference 
between the quantity of water which entered and the amount of evaporation and 
absorption, this interior sea had disappeared, leaving the chotts as an evidence of 
the former condition of things; that, in fact, this was none other than the cele- 
brated Lake Triton, the position of which has always been a puzzle to geographers. 
This theory, however, is untenable ; the Isthmus of Gabés is not a mere sandbank ; 
