s which have
the same winter temperature, they are found to deviate from the
terrestrial parallels much farther than the lines of equal mean annual
heat. The lines of equal winter in Europe, for example, are often curved
so as to reach parallels of latitude 9 degrees or 10 degrees distant
from each other, whereas the isothermal lines, or those passing through
places having the same mean annual temperature, differ only from 4
degrees to 5 degrees in Europe.

_Influence of currents and drift ice on temperature._--Among other
influential causes, both of remarkable diversity in the mean annual
heat, and of unequal division of heat in the different seasons, are the
direction of currents and the accumulation and drifting of ice in high
latitudes. The temperature of the Lagullas current is 10 degrees or 12
degrees Fahr. above that of the sea at the Cape of Good Hope; for it
derives the greater part of its waters from the Mozambique channel, and
southeast coast of Africa, and from regions in the Indian Ocean much
nearer the line, and much hotter than the Cape.[170] An opposite effect
is produced by the "equatorial" current, which crosses the Atlantic from
Africa to Brazil, having a breadth varying from 160 to 450 nautical
miles. Its waters are cooler by 3 degrees or 4 degrees Fahr. than those
of the ocean under the line, so that it moderates the heat of the
tropics.[171]

But the effects of the Gulf stream on the climate of the North Atlantic
Ocean are far more remarkable. This most powerful of known currents has
its source in the Gulf or Sea of Mexico, which, like the Mediterranean
and other close seas in temperate or low latitudes, is warmer than the
open ocean in the same parallels. The temperature of the Mexican sea in
summer is, according to Rennell, 86 degrees Fahr., or at least 7 degrees
above that of the Atlantic in the same latitude.[172] From this great
reservoir or caldron of warm water, a constant current pours forth
through the straits of Bahama at the rate of 3 or 4 miles an hour; it
crosses the ocean in a northeasterly direction, skirting the great bank
of Newfoundland, where it still retains a temperature of 8 degrees above
that of the surrounding sea. It reaches the Azores in about 78 days,
after flowing nearly 3000 geographical miles, and from thence it
sometimes extends its course a thousand miles farther, so as to reach
the Bay of Biscay, still retaining an excess of 5 degrees above the mean
temperature of