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 ivest ; 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. 



