26 THE OCEAN WORLD. 
81”, is much greater than, according to the same laws, it ought to be 
—the observed difference being ‘oors, whereas it ought to be ‘oo25. 
Let us inquire,” he adds, “what makes the equatorial waters so 
much heavier than they ought to be. 
“The anomaly occurs in the trade-wind region, and is best 
developed between the parallel of 40° in the North Atlantic and the 
Equator, where the water grows warmer, but not proportionally 
lighter. The water sucked up by the trade-winds is fresh water, and 
the salt it contained, being left behind, is just sufficient to counteract 
by its weight the effect of thermal dilatation upon the specific gravity 
of water between the parallels of 34° north and south. The thirsting 
of the trade-winds for vapour is so balanced as to produce perfect 
compensation ; and a more beautiful instance than we have here 
stumbled upon is not, it appears to me, to be found in the mechanism 
of the universe.” 
The oceanic currents are due to a great number of causes: the 
duration and force of the winds, for instance; the rise and fall of 
tides all over the globe; the variations in the density of the water, 
according to its temperature and the evaporating powers of the 
atmosphere; the depth and degree of saltness to which we have 
already alluded ; finally, to the variations of barometric pressure. 
The currents which furrow the ocean often present a striking 
contrast with the immobility of the neighbouring waters ; they may 
be seen to form rivers of a determinate breadth, whose banks are 
formed by the water in repose, and whose course is often made quite 
perceptible by stray floating particles, and often by several aquatic 
plants which follow in their train. 
In order to comprehend the origin of these pelagic rivers, it is 
necessary to consider the laws which govern the atmospheric currents, 
in particular the ¢vade winds. ‘‘ Hence,” says Maury, “in studying 
the system of oceanic circulation, we set out with the very simple 
assumption, that from whatever part of the ocean a current is found 
to run, to that same part a current of equal volume is bound to 
return; for on this principle is based the whole system of currents 
aud counter currents.” The differences of temperature between 
equinoctial and polar countries generate two opposing currents, the 
upper one proceeding from the Equator to the Poles, the lower one 
directed from the Poles towards the Equator. On reaching the 
Equator, the cold current of air from the Poles is warmed and 
rarefied, and ascends to the upper beds of the atmosphere, whence it 
is again led to its point of departure; there it is again cooled, and 
returns with the lower current towards the tropical regions. But the 
