at 



i 





Ch. XX.] 



CAUSES OF CURRENTS. 



497 



Ocean at Panama, and of the Atlantic at the mouth of the 

 liver Chagres, have shown, that the difference of mean level 

 between those oceans is not considerable, and, contrary to 

 expectation, the difference which does exist is in favour of 

 the greater height of the Pacific. 



A fourth cause of currents is the discharge into the sea of 

 large bodies of fresh water by the great rivers of the globe, 

 which, as General Sabine remarks, sometimes preserve their 

 original direction and flow with a very slowly diminishing 

 velocity for several hundred miles over the surface of the 

 ocean. Thus he found in the year 1822, that the river 

 Amazons preserved a velocity of nearly three miles an hour, 



at a distance of upwards of 300 miles from its mouth, its 

 original direction being scarcely altered, and the fresh water 

 having only become partially mixed with that of the ocean. 

 The river Plate, says Pennell, has still a velocity cf a mile 

 an hour, and a breadth of more than 800 miles, at a distance 

 of not less than 600 miles from its mouth. 



The existence of another or a fifth class of currents was 

 first suspected when it was observed that the tropical seas 

 had a much lower temperature at great depths than at 

 the surface. Whenever in the arctic or antarctic regions 

 the superficial waters are cooled, provided they do not ap- 

 proach the freezing point, they are rendered heavier by con- 

 densation and fall to the bottom. Lighter water then rises 

 to take their place and by this circulation of ascending and 

 descending currents in high latitudes, the inferior parts of the 

 sea become heavier than the adjoining parts of the temperate 

 and tropical ocean at corresponding levels. In that case, if 

 there be a free communication, if no continuous shoal or 

 chain of submarine mountains divide the polar from the 

 equatorial basins, a horizontal movement will arise by the 

 flowing of the colder water from the poles to the equator, 

 and there will be a reflux of warmer superficial water from 

 the equator to the poles. A well-known experiment has been 

 adduced to elucidate this mode of action in explanation of 

 the ' trade winds/ * If a long trough, divided in the middle 



* See Capt, B. Hall, On Theory of 

 Trade Winds, Fragments of Voy. second 



series, vol. i., and Appendix to Daniel? s 

 Meteorology. 



VOL. I. 



K K 



