THE OCEAN. 6G9 



ig less than one mile an hour. Kane, while shut up in the Arctic, was carried south by 

 the current, some days, about half a mile an hour. The great oceanic current of the 

 eastern South Pacific varies from three miles an hour to a fraction of a mile; and across 

 the middle of the ocean it is barely appreciable. The current in the Indian Ocean, 

 where most rapid, has the hourly rate of two miles and a quarter. 



In past geological ages, the rapidity of these great oceanic currents 

 must have been less than now, if there was any difference, because 

 of the less difference of temperature between the equator and the 

 poles, and hence feebler trade-winds. 



2. The currents are generally remote from coasts, and are seldom 

 appreciable where the depth is less than one hundred feet, and very feeble 

 where less than one hundred fathoms. — Owing to the depth of the 

 oceanic movement, the waters are diverted along the borders of the 

 oceans by the deep-sea slopes of the continents. The Gulf Stream 

 approaches the coast at Cape Florida, and somewhat nearly at Cape 

 Hatteras and Cape Cod ; but, off New Jersey, its western limit is eighty 

 to one hundred miles distant, along the limits of the deep-sea slope. 

 It does n^t reach into the interior of the Mexican Gulf. 



The polar or Labrador current, which is mostly a sub-current, comes 

 to the surface along the same slope, west of the limit of the Gulf 

 Stream, and is slightly apparent on the shore-plateau, but rather by its 

 temperature than by the movement of the waters. The more western 

 position of the limit of the polar current is explained on page 39. The 

 fact that it has not more rapid movement on the great shore-plateau 

 is evidence that it belongs to the deep water. This appears, further, 

 in the current's underlying the Gulf Stream, and its banding the 

 stream with colder and warmer waters, as shown by the Coast Survey, 

 under Professor Bache. The observations of the survey have proved 

 that there are mountain-ridges apparently parallel with the Appala- 

 chians, along the course of the stream, in its more southern part, off 

 the Carolinas, and that, above these ridges, the surface-waters are 

 cooler, owing to the lifting upward of the polar current by the subma- 

 rine elevations. 



Where the current flows close along a coast or submarine bank, or 

 by an oceanic island, it may produce some eroding effects. 



3. As the position of the main flow of the currents is determined 

 partly by the trend of the continents, their courses may have been differ- 

 ent in former time from what they are now, provided the continents, or 

 large portions of them, were sufficiently submerged. — Small subsidences 

 would not suffice to produce a diversion from their present courses, for 

 the reason just given. Even the barrier of Darien might be removed, 

 by a submergence to a depth of five hundred feet, and probably one 

 thousand, without giving passage to much, if any, of the Gulf Stream. 

 If, however, the isthmus were so deeply sunk that the Gulf Stream 



