NARRATIVE OF THE CRUISE. 
967 
is particularly well shown by comparing the North Atlantic with the North Pacific. 
The mean specific gravity of the water between the surface and a depth of 1500 fathoms, 
at lat. 30° 22' N. and long. 154° 56' W., in the Pacific, as determined from observa- 
tions made at nine different depths on the 21st July 1875, was 1 ‘02534, the depth 
being 2950 fathoms; at lat. 26° 21' N. and long. 33° 37' W., in the Atlantic, the mean 
specific gravity of the same stratum was 1 ‘02659, from observations at nine different 
depths on the 3rd May 1876, the depth being 2700 fathoms. The mean temperature 
of the water down to 1500 fathoms, for the Pacific Station was 40°‘16, and for the 
Atlantic one 45°‘53. From observations made in the “ Porcupine,” in lat. 48° N. a 
mean temperature of 43°‘51 was found down to 1500 fathoms, and in lat. 55° 40' N. 
a mean temperature of 43°‘ll down to the same depth; 'it is, therefore, in every way 
likely that a high specific gravity prevails also down to the bottom. In treating of 
the concentration of the North Atlantic, it has already been mentioned that that 
ocean is the recipient of all the brine from the Mediterranean, where evaporation 
goes on with great vigour. Notwithstanding the great supply of fresh water from 
the numerous European rivers and the Nile, which is constantly pouring into it, and 
the rain which falls on itself, there is a constant deficiency in the amount of water 
present in it's basin. That this is so, is evident from the existence of a constant 
inflow through the Strait of Gibraltar at the surface and southern side, and outflow at 
the bottom and northern side of very salt water, both streams being, however, modified 
by tidal currents. The Mediterranean and Eed Seas furnish instructive examples of 
what ocean water tends to become when subjected in the first case to an average climate 
similar to what prevails in the trade wind regions of the North Atlantic, and in the case 
of the Red Sea to a climate of almost perfect dryness and great heat, both being connected 
with the open ocean, so as to admit of interchange of water. The Baltic and Black 
Seas afford examples of similar seas in colder and damper climates, while the Dead 
Sea and the depression in which it lies show the result of completely isolating a drain- 
age area in a very dry climate. The water of the Dead Sea is saturated, containing 
as much as 28 per cent, of salt. The water of the Red Sea contains 4 per cent, of 
salt, and that of the Mediterranean 3 ‘6 to 3 ‘9 per cent., thus showing the freshening 
effect of communication with the ocean. 
It is worthy of remark that in the Norwegian Sea, which stretches from the Faeroe 
Islands to Spitzbergen, and attains a depth of 2500 fathoms, the deep water was found 
to be distinctly of Atlantic origin, both on account of its high salinity and of the small 
amount of dissolved nitrogen which it contained. The comparatively very salt water of 
the Atlantic is rendered less dense by its high temperature, and is thus able to penetrate 
into the Arctic regions, where, on being cooled by the winter climate, it sinks down, raising 
the salinity of the Arctic water with which it mixes, and at the same time carrying down 
to the bottom approximately the temperature of freezing sea water. The distribution of 
(narr. chall, exp,— vol, i. — 1885.) 122 
