820 THE POPULAR SCIENCE MONTHLY. 



equalizing temperature and distributing moisture over the earth 

 is remarkable. If the whole quantity of moisture in the air at 

 any moment were condensed so as to leave it absolutely dry, the 

 resulting stratum of water if distributed evenly over the whole 

 earth would be less than one inch in depth. Yet it is estimated 

 (though perhaps on insufficient data) that the mean rainfall over 

 the whole globe is not less than sixty inches in the year, and falls 

 of ten times this amount are known to occur in some localities. 

 Observations of the velocity of the wind at marine stations show 

 that these results are due to the almost unceasing passage of air 

 highly charged with vapor over the regions where and during the 

 time in which rain thus falls, and to the unceasing renewal of the 

 supply of moisture by evaporation. The relatively very large 

 sea-area has an important effect in maintaining the supply of the 

 rain that falls on the land; and the immediate dependence of 

 rainfall on local geograj^hical features is too well known to call 

 for more than a passing remark. 



A few words will indicate the magnitude of the forces which 

 are called into silent and comparatively unobserved operation in 

 the atmosj)here by the sun's heat in the production and reconden- 

 sation of aqueous vapor. It has, as I noticed, been estimated that 

 on the average five feet of water falls annually as rain over the 

 whole earth. Supposing that condensation takes place at an 

 average height of 3,000 feet above the surface, the force of evap- 

 oration must be equivalent to a power capable of lifting five feet 

 of water, over the whole surface of the globe, 3,000 feet during the 

 year. This, not reckoning the force required for the transport of 

 the rain in a horizontal direction, would involve lifting 332,000,- 

 000 pounds of water 3,000 feet in every minute, which would re- 

 quire about 300,000,000,000 horse-power constantly in operation. 

 Of the huge energies thus exerted a very small part is transferred 

 to the waters that run back through rivers to the sea, and a still 

 smaller fraction is utilized by man in his water-mills ; the remain- 

 der is dissipated in celestial space. A well-known consequence of 

 the physical properties of the air is the gradual reduction of tem- 

 perature observed in ascending mountains. This, amounting to 

 1° for about 300 feet of elevation, gradually produces a change of 

 conditions similar to that caused by passing from the equator 

 toward the poles, and at the greatest elevations an arctic climate 

 is established even under a tropical sun. Among the sublimest 

 sights furnished by nature are the great ranges of mountains 

 which traverse or approach the tropics. Rising into the regions 

 of perpetual snow, they discharge important functions in the 

 economy of the globe. By the intrusion of the solid terrestrial 

 surface into the upper part of the atmosphere, the low tempera- 

 ture there, which otherwise could have x)roduced no effect on the 



