30 
THE VOYAGE OF H.M.S. CHALLENGER. 
throws into the Atlantic a body of water much salter, and therefore, even after allowing 
for temperature, much denser, than the water of that ocean. 
At 200 fathoms to the east of the Strait the temperature is 55°, from which, on 
proceeding eastwards, it gradually rises to 57° about long. 20° E. and to 58° in the 
Levant. To the east of the Strait the specific gravit}^ is now 1*0287, while a little to 
the west in the Atlantic it is 1*0268 ; in the Levant the specific gravity is L0290. At 
300 fathoms, and down to the bottom of the sea, the temperature of the western basin 
is 55°*5 and of the eastern basin 56°*5 ; the mean specific gravity is 1*0290, calculated 
from the numerous observations made in the eastern basin by the “ Pola” Expeditions in 
1890—92. From the few deep-sea observations made of specific gravities in the western 
basin, it is probable that the mean there is about 1*0286. The greatest depth observed 
by the “ Pola” was 4400 metres, or 2406 fathoms, in lat. 35° 45' N. and long. 21° 46' E., 
the temperature at that depth being 56°*3 and the specific gravity 1*0295. The above 
temperature is fully 20° higher than in the open ocean at that depth, and is closely 
approximate to the temperature of the Atlantic adjoining the Strait of Gibraltar at the 
depth of the ridge separating the two seas, or nearly 200 fathoms. 
Owing to the great specific gravity of this warm undercurrent from the Mediterranean, 
it gradually sinks on entering the Atlantic, but its effect in heating the waters of 
this ocean over any considerable extent only becomes clearly apparent at the depth 
of 500 fathoms. Beyond this depth, its great influence is felt over nearly the whole 
breadth of the Atlantic to at least about 1000 fathoms. Periodical surveys of the 
Mediterranean would serve many important objects ; thus, if made every five years, it 
would soon be shown whether the outflow of the Gibraltar undercurrent of such high 
specific gravity is attended with any diminution of the salinity of this sea. It is 
therefore not winds and currents, but different specific gravities and different levels, 
which rule the interchanges of the waters of this sea. 
The Red Sea . 
The mean annual temperature of the surface waters near the head of the sea is 77°; 
it rises to 80' about lat. 22° N., to 84° in lat. 16° N., and then drops down to 82° at the 
Straits. The isothermal of 80° is in long. 50° E. 
At the Strait of Bab-el-Mandeb the specific gravity is 1*0275, which quickly increases 
on advancing northward to 1*0280, 1*0285, 1*0290, 1*0295, 1*0300, 1*0305, and near 
Suez to 1*031. These specific gravities are caused by the rapid evaporation which takes 
place on account of the great heat, dryness, and rainless character of the climate of this 
arm of the sea. 
At 100 fathoms the temperature near the north end of the sea is 65°, from which 
