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Parr Il. Sxor.i] ATMOSPHERIC ACTION. 317 
imply that anything useful to man is destroyed, reproduced, or 
preserved. On the contrary, the destructive action of the atmosphere 
may coyer bare rock with rich soil, while its reproductive effects 
may bury fertile soil under sterile desert. Again, the conser- 
yative influence of vegetation has sometimes for centuries retained as 
barren morass what might otherwise have become rich meadow or 
luxuriant woodland. The terms, therefore, are used in a strictly 
geological sense, to denote the removal and re-deposition of material, 
and its agency in preserving what lies beneath it. 
Section i.— Air. 
The geological action of the atmosphere arises partly from its 
chemical composition and partly from its movements. The com- 
position of the atmospheric envelope has been already discussed 
(p. 80), and further information will be found under the head of Rain. 
The movements of the atmosphere are due to variations in the 
distribution of pressure or density, the law being that air always 
moves spirally from where the pressure is high to where it is low. 
Atmospheric pressure is understood to be determined by two causes, 
temperature and aqueous vapour. Since warm air, being less dense 
than cold air, ascends, while the latter flows in to take its place, the 
unequal heating of the earth’s surface, by causing upward currents 
from the warmed portions, produces horizontal currents from the 
surrounding cooler regions inwards to the central ascending mass of 
heated air. The familiar land and sea breezes offer a good example 
of this action. Again the density of the air lessens with increase 
of water-vapour. Hence moist air tends to rise as warmed air 
does, with a corresponding infiow of the drier and consequently 
heavier air from the surrounding tracts. Moist air, ascending and 
diminishing atmospheric pressure, as indicated by the fall of the ~ 
barometer, rises into higher regions of the atmosphere, where it 
expands, cools, condenses into visible cloud and into showers that 
descend again to the earth. 
Unequal and rapid heating of the air, or accumulation of aqueous 
vapour in the air, and possibly some other influences not yet properly 
understood, give rise to extreme disturbances of pressure, and 
consequently to storms and hurricanes. For instance, the barometer 
sometimes indicates in tropical storms a fall of an inch anda half in an 
hour, showing that somewhere about a twentieth part of the whole mass 
of atmosphere has in that short space of time been displaced over a 
certain area of the earth’s surface. No such sudden change can occur 
without the most destructive tempest or tornado. In Britain the 
tenth of an inch of barometric fall in an hour is regarded as a large 
amount, such as only accompanies great storms.’ The rate of move- 
ment of the air depends on the difference of barometric pressure 
between the regions from and to which it blows. Since much of the 
potency of the air as a geological agent depends on its rate of 
* Buchan’s Meteorvlogy, p. 266. ’ 
