42 



THE OPAL SEA 



Circulation 

 of the seas. 



" Horizontal circulation " is the name given 

 to the movement of the well-defined surface 

 currents; whereas the creep of scattered bodies 

 of water up and down, here and there, is known 

 as the " vertical circulation." Possibly the one 

 is but a slower manifestation of the other, and 

 perhaps both have causes, aside from the steady 

 blowing Trades, that contribute to the total 

 result. One of these causes is undoubtedly dif- 

 ference in gravity. The seas of the tropics, for 

 instance, are subject to vastly more evapora- 

 tion than those of the polar regions. There is 

 more evaporation than rainfall, and, conse- 

 quently, an ever-growing gravity and decreased 

 bulk. At the poles, on the contrary, with much 

 snow and ice continually melting and very lit- 

 tle evaporation, there must be a tendency to 

 freshness. This means decreased gravity and 

 increased bulk. 



It is not possible for this difference between 

 the polar and the equatorial waters to exist 

 without an attempt at equalization. The great 

 Leonardo argued that much long before the 

 seas were charted or a theory of gravitation was 

 advanced. The result of the attempt at equali- 

 zation is an exchange, an interchange. The 

 waters of the north and south work underneath 

 by current, drift, and gradual " creep '' toward 



Exchange of 

 currents. 



