26 THE OC&AH WORLD. 



81, is much greater than, according to the same lav^, 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 

 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 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 

 return ; for on this principle is based the whole system of currents 

 and 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 



