248 THKEE CKUIbES OF THE '* BLAKE." 



movements of our atmosjjhere, be dejDendent upon cosmic phe- 

 nomena, practically independent of any secondary causes^ and 

 modified by them within very narrow limits ? 



The difference in salinity of certain oceanic districts is in it- 

 self insufficient to explain oceanic circulation ; so that while the 

 secondary causes referred to above are undoubtedly active as 

 producing more or less extensive local circulation, we seem jus- 

 tified in looking upon the differences of temperature of the zones 

 of the ocean as the principal cause of the general oceanic cir- 

 culation. We may state, in the maiu, that the density of the 

 ocean water is least at the equator, gradually rises towards the 

 poles, and attains its maximum at 60'' of latitude. For the sake 

 of convenience we may call the density of the ocean as one at a 

 depth of 500 fathoms, and consider the strata of water above 

 and below as having a less and a greater density,^ within very 

 narrow limits; thus the watery envelope is not in a state of 

 equilibrium. 



The most important disturbing factors of a uniform distribu- 

 tion of oceanic temperature are the continental masses which lie 

 in the path of the equatorial currents. A comparison of the 

 position of the oceanic isotherms of the North and South At- 

 lantic shows a striking contrast in their course north and south 

 of the equator. A similar comparison between the Atlantic 

 and Pacific brings out plainly the contrast in the course of the 

 isotherms of two oceans, in which the disturbing effect is due 

 in the one to continental masses, and in the other to large 

 groups of oceanic islands. 



Perhaps the best example of the unstable equilibrium existing 

 between adjoining oceanic areas is furnished by the heaping up 

 of the waters driven by the tradewinds into the Gulf of Mexico 

 from the Caribbean. The amount of this accumulation has 

 actually been measured by officers of the United States Coast 

 Survey. It gives an additional force at work to keep up the 

 efficiency of the Gulf Stream. The Gulf of Mexico is consid- 



^ Ocean water, at depths exceeding either colder or warmer, it must expand ; 



1,000 fathoms, has a temperature of this it cannot do, on account of the pres- 



nearly 35° F., the temperature of great- sure, 

 est density. Should the water become 



