288 THE BOTTOM OF THE SEA 



slow movement of the same wheel would not bo 

 observed. 



The air is composed of a great number of elements. 

 Oxygen and nitrogen form the greater part of its 

 composition. The vapour of water, carbonic acid gas, 

 and every kind of emanation from the earth's surface, 

 add themselves to the two first-named gases. Oxygen 

 is absorbed by animals, and replaced by carbonic 

 acid. The converse takes place in vegetation, and a 

 like exchange of gases occurs in combustion. 



The vapour of water, raised by the action of the 

 winds from the vast oceanic reservoirs, floats above us 

 in the form of clouds, and is distributed over the 

 surface of the earth in beneficent showers. But 

 again the water finds its way back by means of 

 rivers and floods to its original point of departure, to 

 be again converted into vapour and recommence the 

 cycle of its transformations. 



There is a compensating or balancing process 

 continually going on among the numerous causes of 

 change, which renders the composition of the air 

 sensibly constant, and makes its every movement 

 tend to uniformity. 



The cloud which obscures the summit of a moun- 

 tain marks the locality of a very rapid movement of the 

 air. This motion is caused by the contact of a hot 

 and moist air with one much colder. But the wind bears 



