THE GULF STREAM. 
35 
31. There are physical agents that are known to be at work in 
different parts of the ocean, the tendency of which is to make the 
waters in one part of the ocean Salter and heavier, and in another 
part lighter and less salt than the average of sea water. These 
agents are those employed by sea-shells in secreting solid matter 
for their structm'es, also of heat* and radiation, evaporation and 
precipitation. 
32. In the trade-wind regions at sea (Plate VIIL), evaporation 
is generally in excess of precipitation, while in the extra-tropical 
regions the reverse is the case ; that is, the clouds let down more 
water than the winds take up again ; and these are the regions in 
which the Gulf Stream enters the Atlantic. 
33. Along the shores of India, where experiments have been 
carefully made, the evaporation amounts to three fourths of an 
inch daily. Suppose it in the trade-wind region of the Atlantic 
to amount to only half an inch, that would give an annual evap- 
oration of say fifteen feet. In the process of evaporation from the 
sea, fresh water only is taken up, the salts are left behind. 
Now a layer of sea water fifteen feet deep, and as broad as the 
trade-wind belts of the Atlantic, and reaching across the ocean, 
contains an immense amount of salts. 
34. The great equatorial current (Plate VI.) which sweeps from 
the shores of Africa across the Atlantic into the Caribbean Sea 
is a surface current ; and may it not bear into that sea a large 
portion of those waters that have satisfied the thirsty trade-winds 
with saltless vapor? If so — and it probably does — have we not 
detected here the foot-prints of an agent that does tend to make 
the waters of the Caribbean Sea Salter, and therefore heavier than 
the average of sea water ? 
It is immaterial, so far as the correctness of the principle upon 
which this reasoning depends is concerned, whether the annual 
evaporation from the trade-wind regions of the Atlantic be fifteen, 
ten, or five feet. The layer of water, whatever be its thickness, 
that is evaporated from this part of the ocean, is not all poured 
back by the clouds in the same place w^hence it came. But they 
take it and pour it down in showers upon the extra-tropical regions 
of the earth — on the land as well as in the sea — where, as a rule, 
more water is let down than is taken up into the clouds again. 
* According to Doctor Marcet, sea water contracts down to 28°, 
