52 Currents of the Ocean. 



on account of the steady increase of the diameter of rotation 

 between the Pole and the parallel of lat. 30 , will, after crossing 

 that parallel, gradually lose that tendency, and will, within 

 io° of the Equator, have acquired the rotatory velocity of the 

 earth's surface, and therefore cease to show itself as a westerly 

 current. 



It thus appears that the Equator, the parallels of lat. 30 , 

 and the Poles, constitute what may be termed the critical 

 latitudes in the system of atmospheric circulation, and for similar 

 reasons also in the system of oceanic circulation. 



This state of matters, according to which we find the surface 

 of our planet, as regards its two fluid envelopes, divided into 

 belts of calms and belts of currents symmetrically distributed 

 on each side of the Equator, is subject to considerable modifi- 

 cations from various causes. The first and most important of 

 these causes is the division of the surface of our planet into 

 areas of land and water which, alternately stretching across the 

 Equator from one hemisphere into the other, intersect the 

 parallel belts of calms and of currents at right angles. We 

 have here carried out on a large scale one of those simple ex- 

 pedients by which Nature, in strict obedience to her laws, creates 

 that endless variety of contrasting phenomena, which the philo- 

 sopher, the poet, the artist, never cease to behold with wonder, 

 and which, while it is the source of all beauty, is, at the same 

 time, a necessary condition to the existence of all life. The 

 result, in the present case, is the creation of numerous areas of 

 atmospheric and oceanic circulation corresponding with the 

 different areas of land and of water distributed on each side 

 of the Equator, and the subdivision of the belts of calms into 

 distinct areas of calms, of which we find one in each of the 

 oceanic basins, in the North and South Atlantic, in .the North 

 and South Pacific, and in the Indian Ocean. (Plate 4 A.) 



A comparison of these areas of calms with a chart of 



