28 THE OCEAN WORLD. 



of the compensations by which the oceanic waters are governed : 

 " According to Eodgers' observations," he says, " the average specific 

 gravity of sea water on the parallels of thirty-four degrees north and 

 south, at a mean temperature of sixty-four degrees, is just what it 

 ought to be, according to saline and thermal laws ; but its specific 

 gravity, when taken from the Equator at a mean temperature of eighty- 

 one degrees, is much greater than, according to the same laws, it ought 

 to be the observed difference being '0015, whereas it ought to be 

 0025. 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 de- 

 veloped between the parallel of forty degrees 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 thirty-four degrees north and south. 

 The thirsting of the trade-winds for vapour is so balanced as to pro- 

 duce perfect compensation, and a more beautiful instance than we have 

 here stumbled upon is not, it appears to me, to be found in the mecha- 

 nism 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 waters, 

 according to its temperature, and the evaporating powers of the atmo- 

 sphere; 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 present a striking contrast with 

 the immobility of the neighbouring waters; they form rivers of a 

 determinate breadth, whose banks are formed by the water in repose, 

 and whose course is often made quite perceptible by the vrachs and 

 other 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 trade-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 



