196 W J MCGEE OUTLINES OF HYDEOLOGY 



by cooling the latent heat tempers the atmosphere and lithosphere, and 

 when warmed it returns to vaj)or, and the latent heat again equalizes the 

 temperature; and through this conservation and equalization of heat de-. 

 rived primarilj' from the sun, the range of temperature from equator to 

 poles, from day to night, and from summer to winter is reduced to a third 

 or a fifth of what it would be if dependent directly on insolation and 

 radiation. Despite the diathermancy of the air, the atmosphere as a 

 whole is so effective an agency for retaining sun heat that the temperature 

 of the external earth is conditioned not (almost not at all) by the planet's 

 proper heat, but by radiation from the distant luminary; and the chief 

 heat-conserving constituent is. water, mainl}' in the form of vapor, of 

 which each particle, as shown by Tjmdall, absorbs heat many thousand 

 times more rapidly than a like particle of air. With an oceanless surface 

 and a water-free atmosphere, the temperature of the earth would range 

 so widely and fall so low as to render it uninhabitable by organized 

 beings. 



THE HYDROSPHERE AND ITS DIVISIONS 



The volume of the hydrosphere is about 1/600 that of the entire geo- 

 sphere. The dominant division of the terrestrial HjO (some 310,000,000 

 cubic miles, or 1/840 of the volume of the solid earth) occupies depres- 

 sions in the earth crust as seas. Probably next in volume is the division 

 permeating earth and rocks as ground water: if 5 per cent of the ten- 

 mile earth shell (Whitney finds the optimum soil moisture to range from 

 4 per cent to 20 per cent, which increases at ground-water depths to an 

 average exceeding 15 per cent in humid regions, but runs lower in arid 

 regions and generally at greater depths), it reaches some 100,000,000 

 cubic miles. The third division in magnitude may be that interpene- 

 trating the atmosj)here in gaseous form; the mean annual precipitation 

 is about 120,000 cubic miles — 30,000 on land and 90,000 on the oceans, 

 or some 10 per cent of the volume of the atmosphere reduced to liquid 

 measure — which ma}^ be regarded as a rough indication of the average 

 content of aqueous vapor. The next smaller division is that gathered 

 largely in polar regions and elevated areas in the solid form (snow, neve, 

 ice) ; its volume can not well be estimated. The smallest primary divis- 

 ion, albeit the most vigorous as an agency of earth-making, is the moving 

 water of streams and springs, sometimes lodging temporarily in ponds 

 and lakes; its volume is negligible in proportion to that of the oceans. 



The sea water is a seat of chemical action, including that involved in 

 the development of primordial life, and through both waves and tides an 

 agency of mechanical action in sapping shores and distributing sedi- 





