PRELIMINARY OUTLINE. 7 



conditions for water action. Partly by its mechanical aid, but chiefly 

 by securing the right temperature, it is a necessary factor in the action 

 of rains, streams, glaciers, and the various forms of moving water upon 

 land. So also, on the ocean, wave action is essentially dependent on 

 the winds. In the absence of atmospheric propulsion, wave action 

 would be chiefly confined to the tides and to occasional earthquake im- 

 pulses, and would lose nearly all its efficiency. Stream action and wave 

 action, which are the most declared of the geological agencies, are there- 

 fore to be credited as much to the atmosphere as to the hydrosphere, 

 since the action is a joint one to which both envelopes are essential. 



A thermal blanket. — A function of the atmosphere of supreme 

 importance is the thermal blanketing of the earth. In its absence 

 the heat of the sun would reach the surface with full intensity, and 

 would be radiated back from the surface almost as rapidly as received, 

 and only a transient heating would result. During the night an in- 

 tensity of cold would intervene scarcely less severe than the temperature 

 of space. In penetrating the atmosphere certain portions of the 

 radiant energy of the sun are absorbed. Of the remainder which 

 reaches the surface of the earth, a part is transformed into vibrations of 

 lower intensit}^, which are then more effectively retained by the atmos- 

 phere. The air thus distributes and equalizes the temperature. The 

 two constituents of the atmosphere which are most efficient in this 

 work are aqueous vapor and carbon dioxide, and the climate of the 

 earth is believed to have been very greatly affected by the varying 

 amounts of these constituents in the atmosphere, as well as by the 

 total mass of the atmosphere. 



The function of the atmosphere in sustaining life and promoting 

 all that depends on life is too obvious to need comment. 



The special geological action of the atmosphere will be discussed 

 in the next chapter. 



//. The Hydrosphere. 



About 1300 quadrillion tons of water lie upon the surface of the 

 solid earth. This equals about t-^Vo" part of the earth's mass. Were 

 the surface of the soUd earth perfectly spheroidal, this would constitute 

 a universal ocean somewhat less than two miles deep. Owing to 

 the inequalities of the rock surface, the water is chiefly gathered into 

 a series of great basins or troughs occupying about three-fourths (72%) 



