46- PHYSIOGRAPHIC GEOLOGY. 



narrow the areas on the east side. The cohl or polar latitudes, as has 

 been explained, send a returning current along the continental borders 

 equatorward, which may be stronger on the eastern or western border, 

 according to geographical conditions, and thus these cold waters may mod- 

 ify the temperature and position of the currents in the warmer latitudes. 

 Thirdly, owing to the effect of the more rapid flow of the current along the 

 borders of the continents, the currents often carry the isothermals polcAvard, 

 making poleward bends or loops in their courses ; and these may be greatly 

 increased in prominence or definition by the polar current along the con- 

 tinental borders. 



In Fig. 27, the elliptical line (A'B'AB) represents the course of the current in an 

 ocean soutli of the equator (EQ). If now tlie movement in the circuit were equable, 



an isothermal line, as that of 68°, would extend obliquely 



^' • across, as nn : it would be thrown south on the west side 



of the ocean by the warmth of the torrid zone, and north 



y^ ^ ^"7^ *^^ *^® ®^^* ^^^® ^y *^® cooling influence derived from its 



/ \^ \n flow in the cold-temperate zone. But if the current, in- 



// r ^^--'^r^'' 1 a stead of being equable throughout the area, were mainly 



sv-_*'--'j6-^''^^ / apparent near the continents (as is actually the fact) , the 



'f \ '■■.,. y\ isothermal line should take a long bend near the coasts, 



• r'^^--.^ ^ B ^ __^^ \ as in the line A'r'n-rrA, or a shorter bend A'ss', ac- 



' Oceanic Currents, D. '58. '' cording to the nature of the current. This form of the 



isothermal line of 68° on the chart indicates the exist- 

 ence of the circuit movement in the ocean, and also some of its characteristics. 



For example, the westward tropical flow in the north Atlantic carries its 

 warm waters over the Bermudas, bending northward the isotherm of 68° 

 (see map, page 47), and also that of 62°; and in the south Atlantic, bending 

 the isotherms of 74° and 68° far away from the equator, the latter to latitude 

 30° ; while on the west side of Europe and Africa, as no tropical flow reaches 

 the borders, and only the high-latitude current, the isotherm of 68° is carried 

 in the north Atlantic to 15° N., and in the south Atlantic up to 6° S. Con- 

 sequently the interval between the isotherms of 68° in the eastern part of the 

 Atlantic Ocean is only 21° in width, while it is 64° in the western. 



The isotherms on the following chart (page 47) mark the points which 

 have equal mean temperature for the coldest winter month, and the tem- 

 peratures are those of a surface layer of the ocean 90 to 180 feet deep. 

 For the northern hemisphere the month of greatest mean cold is January or 

 February, and for the southern, July or August. The chart, while isothermal, 

 differs widely, therefore, from other isothermal charts, and has been named 

 Isocrymal, from the Greek for equal and cold (Lao's, Kpvfjios). The line of 

 68° F., for example, passes through points in which the mean temperature 

 of the surface water in the coldest month of the year is 68° F. ; so with the 

 lines of 62°, 56°, etc. 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, 



