GENERAL FEATURES OF THE EARTH. 41 



Tents tends to distribute tropical heat toward the poles, and polar 

 cold, in a less degree, toward 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 isother- 

 mal 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 lines of 62°, 56°, etc. 1 All of the chart be- 

 tween 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 3 and 62°, 62° and 56°, 56° and 50°, and so on, have special names on the chart. 

 They are as follow : — 



1. Torrid Zone. — Super-torrid, torrid, and sub-torrid regions. 



2. Temperate Zone. — Warm-temperate, temperate, sub-temperate, cold-temperate, 

 and sub-frigid regions. 



3. Frigid Zone. 



They are convenient with reference to the geographical distribution of oceanic 

 species. 



Since the tropical (the westward) currents are warm, and the extra- 

 tropical (the eastward) necessarily cold, the elliptical interplay ex- 

 plained must carry the warm waters away from the equator on the 

 west side of the oceans, and the cold waters toward the equator on the 

 east side. The distribution of temperature thus indicates the currents. 

 In each elliptical circuit, therefore, the line of 68° F. should be an ob- 

 lique diagonal line to the ellipse ; and thus it is in the North Atlantic, 

 the South Atlantic, the North Pacific, the South Pacific (though less 

 distinctly here, as the ocean is so broad), and the Indian Ocean. The 

 torrid-temperature zones are very narrow to the eastward and broad 

 to the westward. The temperate zones press toward the equator 

 against western Africa and Europe, and western America. On the 

 South American coast, this is so marked that a tropical temperature 

 does not touch the whole coast, except near the equator, and does not 

 even reach the Galapagos under the equator off the coast, as shown by 

 the course of the isothermal line of 68°. So, in the South Atlantic, the 

 colder waters extend north to within six degrees of the equator, where 

 the line of 68° leaves the African coast. The continuation of the 

 Gulf Stream up between Norway and Iceland is shown by the great 

 loops in the lines of 44° and 35°. The effect of the Labrador or polar 



1 As the lines are lines of equal extreme cold, instead of heat, such a chart is named 

 an isocrymal chart (from Zero?, equal, and Kpu^os, extreme cold). 



