14 PHYSICAL GEOGRAPHY OF TILE SEA. 
altered mean temperature of those ocean plains than in the 
changed medium warmth of the dry land.” 
The warmest part of the ocean does not coincide with the 
Equator, but seems to form two not quite parallel bands to the 
north and south. 
In the northern Atlantic, the line of greatest temperature (87° 
F.) which on the African coast is found but a little to the north of 
the Equator, rises on the north coast of South America as high 
as 12° N. lat.,and in the Gulf of Mexico ranges even beyond the 
tropic. The influence of the warmth-radiating land on inclosed 
waters is still more remarkable in the Mediterranean (between 
30° and 44° N. lat.) where during the summer months a temper- 
ature of 84° and 85° is found, three degrees higher than the 
medium warmth of the open tropical seas. 
While in the torrid zone the temperature of the ocean is 
generally inferior to that of the atmosphere, the contrary takes 
place in the Polar seas. Near Spitzbergen, even under 80° N. 
lat., Gaimard never found the temperature of the water below 
4+-33°. Between Norway and Spitzbergen the mean warmth of 
the water in summer was +39°, while that of the air only 
attained + 37°. 
In the enclosed seas of the Arctic Ocean, the enormous accu- 
mulation of ice, which the warmth of a short summer is unable 
totally to dissolve, naturally produces a very low temperature of 
the waters. Thus, in Baffin’s Bay, Sir John Ross found during 
the summer months only thirty-one days on which the tempe- 
rature of the water rose above freezing point. 
In the depths of the sea, even in the tropical zone, the water 
is found of a frigid temperature, and this circumstance first led 
to the knowledge of the submarine polar ocean currents; “ for 
without these, the deep sea temperature in the tropics could 
never have been lower than the maximum of cold, which the 
heat-radiating particles attain at the surface.” * 
It was formerly believed that while the surface temperature —- 
which depended upon direct solar radiation, the direction of 
currents, the temperature of winds, and other temporary causes — 
might vary to any amount, at a certain depth the temperature 
was permanent at 4° C., the temperature of the greatest density 
of fresh water. Late investigations, however, have led to the 
* Tumboldt’s “ Kosmos,” 
