42 PHYSIOGRAPHIC GEOLOGY. 



of the South Atlantic and North Atlantic are united, a large part 

 of the tropical current of the former goes to swell the tropical cur- 

 rent and Gulf Stream of the latter. 



In the North Pacific there is the same system, modified mainly 

 by this, that the connection with the polar regions is only through 

 the narrow and shallow Behring Straits. There is a current an- 

 swering to the "Gulf Stream" off Japan, and another corresponding 

 to the "Labrador current" along the whole length of the Asiatic 

 coast, perceptible by the temperature if not by the movement. 



In the South Pacific there are traces of a " Gulf Stream" — that is, 

 of an outward-bound tropical current — off Australia, noticed by 

 Captain Wilkes. The inward extratropical current, chilled by its 

 southern course, is a very important one to western South Ame- 

 rica, as it carries cool waters quite to the equator. 



In the Indian Ocean the system exists, but with a modification 

 depending on the fact that the ocean has no extended northern 

 area. The outward tropical current is perceived off southeast- 

 ern Africa. 



The surface-currents of the ocean are more or less modified by 

 changes in the winds. On this and on other related topics barely 

 glanced at in this brief review the reader may refer to treatises on 

 Meteorology or Physical Geography. 



44. (2.) Oceanic temperature. — The movement of the oceanic 

 currents tends to distribute tropical heat towards the poles, and 

 polar cold, in a less degree, towards the tropics ; and hence the 

 courses of the currents modify widely the distribution of oceanic 

 heat. The chart at the close of this volume contains a series of 

 oceanic isothermal lines drawn through places of equal cold for 

 the coldest month of the year. The line of 68° F., for example, 

 passes through points in which the mean temperature of the water 

 in the coldest month of the year is 68° F. ; so with the line of 62°, 

 56°, &c* All of the chart between the lines of 68°, north and south 

 of the equator, is called the Torrid Zone of the ocean's waters ; the 

 region between 68° and 35°, the Temperate Zone, and that beyond 35°, 

 the Frigid Zone. The line of 68° is that limiting the coral-reef seas 

 of the globe, so that the coral-reef seas and Torrid Zone thus have 

 the same limits. 



The regions between the successive lines, as 80° and 80°, 80° and 74°, 74° and 

 68°, 68° and 62°, 62° and 56°, 56° and 50°, and so on, have special names on 

 the chart. They are as follow : — 



* As the lines are lines of equal extreme cold, instead of heat, such a chart 

 is named an isocrymal chart (from tuog, equal, and Kpv^og, extreme cold). 



