CURRENTS OF THE SEA. 
131 
tilled water by more than four times the usual excess, and accord- 
ingly leaves, upon evaporation, more than four times the usual 
quantity of saline residuum. Hence it is clear that an under cur- 
rent outv^^ard of such denser water, if of equal breadth and depth 
with the current inward near the surface, would carry out as much 
salt below as is brought in above, although it moved with less than 
one fourth part of the velocity, and would thus prevent a perpet- 
ual increase of saltness in the Mediterranean Sea beyond that ex- 
isting in the Atlantic." 
The doctor obtained this specimen of sea water from Captain, 
now Admiral Smyth, of the English navy, who had collected it 
for Dr. Marcet. Dr. Marcet died before receiving it, and it had 
remained in the admiral's hands -some time before it came into 
those of Wollaston. 
It may, therefore, have lost something by evaporation ; for it is 
difficult to conceive that all the river water, and three fourths of 
the sea water which runs into the Mediterranean, is evaporated 
from it, leaving a brine for the under current having four times 
as much salt as the vrater at the surface of the sea usually con- 
tains. Very recently, M. Coupvent des Bois is said to have shown, 
by actual observation, the existence of an outer and under current 
from the Mediterranean. 
However that may be, these facts, and the statements of the 
Secretary of the Geographical Society of Bombay 250), seem 
to leave no room to doubt as to the existence of an under current 
both from the Red Sea and Mediterranean, and as to the cause 
of the surface current which flows into them. I think it a matter 
of demonstration. It is accounted for (§ 245) by the salts of the - 
sea. 
253. Writers whose opinions are entitled to great respect dilFer 
with me as to the proof of this demonstration. Among these 
writers are Admiral Smyth, of the British Navy, and Sir Charles 
Lyell, who also differ with each other. In 1820, Dr. Marcet, be- 
ing then engaged in studying the chemical composition of sea 
water, the admiral, with his usual alacrity for doing " a kind turn," 
undertook to collect for the doctor specimens of Mediterranean 
water from various depths, especially in and about the Straits of 
Gibraltar. Among these was the one 252) taken fifty miles 
within the Straits from the depth of six hundred and seventy 
