6 GEOLOGY. 



Mass and extent. — The total mass of the atmosphere is estimated 

 at fiYQ quadrillion tons, or T2"o7r-o o"o- of the mass of the earth. It is 

 relatively dense at the surface of the earth and decreases in density 

 outwards in a manner difficult of absolute determination, so that the 

 actual height of the appreciable atmosphere is not positively known. 

 The true conception of the atmosphere is perhaps that of a tenuous 

 envelope exerting a pressure of about fifteen pounds per square inch 

 at the sea-level, and thinning gradually upwards until it reaches a 

 tenuity which is inappreciable, but perhaps not ceasing absolutely 

 until the sphere of gravitative control of the earth is passed, about 

 620,000 miles from the lithosphere. In the lower portion, according 

 to the kinetic theory of gases, the molecules fly to and fro, colliding 

 with each other with almost inconceivable frequency, and with very 

 short paths between successive collisions, but in the upper rare portion 

 some of the molecules bound outwards, and do not strike other mole- 

 cules, and hence pursue long elliptical paths until the gravity of the 

 earth overcomes their momentum, when they return, perhaps to bound 

 off again or to force other molecules to do so. This fountain-like 

 nature of the outer part of the atmosphere makes any sharp definition 

 of its limit impracticable. Some molecules are believed to be shot 

 away at such speed that they do not return. Beyond about 620,000 

 miles from the surface of the lithosphere, the differential attraction of 

 the sun is greater than that of the earth, and if the attraction of the 

 earth does not turn the molecules back before reaching this distance, 

 they are almost certain to be lost to the earth. 



The measurement of heights by the aneroid barometer, which is 

 much used in practical geology, is dependent on the lessening of pressure 

 as the instrument is carried upward. 



Geologic activity. — The atmosphere is the most mobile and active 

 of the three great subdivisions of the earth, and when its indirect effects 

 through the agency of water, as well as its direct effects, are considered, 

 it is to be regarded as one of the most effective agencies of change. It 

 acts chemically upon the rock substance of the earth, causing induration 

 in some instances, but more often inducing disintegration and change 

 of composition by means of which rock is reduced to soil, or soil-like 

 materia,lj and rendered susceptible of easy removal by winds and waters. 

 "When in motion the atmosphere acts mechanically on the surface of 

 the earth, transporting dust and sand, and by the friction of these it 

 abrades the surface. It is chiefly effective, however, in furnishing the 



