Page:Popular Science Monthly Volume 33.djvu/840

820 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 geographical 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 atmosphere by the sun's heat in the production and recondensation 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 evaporation 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 require 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 remainder is dissipated in celestial space. A well-known consequence of the physical properties of the air is the gradual reduction of temperature 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 temperature there, which otherwise could have produced no effect on the