GENERAL FEATURES OF THE EARTH. 43 



5. Knowing the temperature limiting the coral-reefs of the pres- 

 ent era, or any species of plants or animals, the geologist has a gauge 

 for comparing the present distribution of temperature and life with 

 the past. 



5. ATMOSPHERIC CURRENTS AND TEMPERATURE. 

 General System. — The system of atmospheric movement has a 

 general parallelism with that of the ocean. In the tropics, the flow is 

 from the east, constituting what are called the trades; in high-tempe- 

 rate latitudes, it is from the west ; and the two pass into one another in 

 mutual interplay. Between these there is, in mid-ocean, a region of 

 calms. The extratropical winds also in part pass on to the poles, 

 to return, as northeast, north, and northwest winds, toward the equa- 

 tor. 



The cause of the motion is not now considered, as it is here in place only to present 

 in a comprehensive manner the earth's exterior features. The causes varying the direc- 

 tions consist in — (1) the temperature of the land and ocean; (2) the form of the land 

 (mountains being barriers to a flow, retarding by friction, etc); (3) difference of density 

 of cold and warm air; (4) changing seasons, etc. But these sources of disturbance 

 only modify without suspending the system of movement. 



Climate. — Climate, while dependent largely on the latitude, is 

 modified by the atmospheric and oceanic movements and the distribu- 

 tion of land and water. A few general facts are here mentioned, in 

 order to complete this survey of the earth's physiography. 



1. The land takes up heat rapidly in summer, and, in the north, be- 

 comes frozen and snow-clad in winter. Land-winds may, consequently, 

 be intensely hot or intensely cold ; and hence lands have a tendency 

 to produce extremes of climate. 



A place on the continents having a mean January temperature of 50° (a very warm 

 temperature for that season) is to be found only in warm latitudes, and one with a mean 

 July temperature of 50° (a cold temperature for the season) only in the colder zones of 

 the globe. The mean January temperature of New York is 31|° F., while the mean July 

 temperature is 73°. Now, in North America, the January isothermal line of 50° almost 

 touches the Gulf of Mexico, and the July line of 50° passes near the mouth of Mac- 

 kenzie River, or the Arctic circle, — the extreme winters and intense summers causing 

 this great change. In Asia, again, the January line of 50° runs just north of Canton, 

 near 26° N., and the July line of 50° touches the Arctic Ocean at the mouth of the Lena, 

 in 72° N., making a difference of 46° of latitude, or nearly 3,000 miles, as the effect of the 

 land on the climate. 



2. The waters of the oceans remain unfrozen even far toward the 

 pole, unless crowded with lands, their perpetual movements tend- 

 ing to produce a uniformity of temperature over the globe ; and 

 hence winds from the oceans or any large body of water are mod- 

 erating, and never very cold. They produce what is called an insular 

 climate. 



