July, 1891. 
THE GULF STKEAM. 
163 
At tlie point of exit of the warm stream from the Gulf of 
Mexico the cold stream passes below it ; the cold water being 
heavier than the warm water (about l-300th part heavier) 
sinks to the bottom, whilst the warm water remains at the 
surface ; and in the Caribbean Sea, adjoining the Gulf of 
Mexico, it has been found that, although the surface tempera¬ 
ture is as high as 85°, the temperature of the water at the 
bottom is only 88°, being a difference of nearly 50° between 
the Arctic Current flowing in from the north and the Equa¬ 
torial Current flowing from the east. 
The Equatorial Current is considered to be caused by the 
Trade Wind, which is constantly blowing from east to west, 
and drags the surface water with it by friction; the friction 
between air and water being very considerable, as is shown 
by the heavy scud that is seen flying in a gale at sea, which 
is water lifted up and carried a long distance by the friction 
of the wind passing over the surface of the sea. The Trade 
Wind follows the sun in travelling round the earth from east 
to west, and arises from its continuous heating action upon 
the surface, and the consequent heating of the air that is in 
contact with that surface. Whatever part the sun is shining 
upon becomes greatly heated, and a heated column of air is 
produced, that rises up in the atmosphere on account of its 
lightness, being made about l-25th part lighter by the 
expansion resulting from this heating. 
This ascending column of heated air has to be replaced 
by an equal amount of air flowing in from all available direc¬ 
tions ; but this can only flow in from the east, north, and 
south, and not from the west, because the sun’s heating 
action is continually advancing westward, and instead of a 
single column of ascending heated air there is really a con¬ 
tinuous east and west wall of heated air constantly ascending, 
following the course of the sun, and merely varying in its 
ascending power with the variations of temperature at each 
point during the 24 hours. The consequence is a continuous 
wind blowing in to the base of this equatorial wall of heated 
air from the east, north, and south ; the north and south 
winds balance one another, and the absence of any wind 
from the west causes the result to be the continuous 
equatorial wind blowing from the east, known as the Trade 
Wind. 
The Gulf of Mexico appears at first rather a small space 
for the remarkable bend in the Gulf Stream that takes place 
there ; but it is really a very large space, being as much as 
1000 miles in length and 700 miles in width, whilst the 
stream there is only about 50 miles in width. 
