KELATIONS OF AIK AND WATER TO TEMPERATURE AND 



LIFE.* 



By Gardiner G. Hubbard, 



President of the National Geographic Society. 



CIRCULATION OF AIR AND WATER. 



It was said in oldeu times, "The ^yiud blowetU where it listeth, and 

 thou hearest the sound thereof, but canst not tell whence it cometh and 

 wbither it goeth." 



That which was unknown, science hath revealed. Tbe wind in its cur- 

 rents is governed and directed by laws as fixed as those of the solar 

 .system. If a moisture-laden wind passes over the country it leaves 

 the land fruitful ; but a dry wind leaves it barren. The currents of air 

 are among the most important factors in the physical geography of our 

 earth, affecting not only soil and climate but also vegetal and animal life. 



The winds obtain their moisture through evaporation, which goes on 

 everywhere and at all times; in the equatorial and polar oceans, from 

 the rich cultivated soil and the arid desert, from the valley and the 

 snow-clad mountain. Reclus tells us that the evaporation from the 

 equatorial ocean is from 13 to 16 feet a year. This estimate is con- 

 firmed by the U. S. Geological Survey, which found the evaporation 

 from the southern Colorado River to be 102 inches, or nearly 9 feet in 

 a year. The quantity of water evaporated from the land must be very 

 large, as only about two-fiiths of the rainfall is returned by the rivers 

 to the ocean. A great part, i^robably more than one-half of this quan- 

 tity, is re-evaporated to fall the second and third time as rain. 



The movements of the atmosphere depend either directly or indi- 

 rectly on differences of temperature ; without these difterences the air 

 and ocean would be stagnant. There is a constant interchange of 

 atmosphere between the equator and the poles. Cool air from the north 

 blows toward the equator, first in a southwesterly, then in a westerly 

 direction, crossing the Atlantic about the tropic of Cancer. Cool air 

 from the south blows in a northwesterly and westerly direction, and 

 crosses the Atlantic near the equator. The difterence of solar acces- 



* Address before World's Cougress at Chicago, July, 1893. From The yational 

 Geographic Magazine, vol. v, pp. 112-124. 



265 



