The air or atmosphere which surrounds the earth is a mixture of two very different gases called oxygen and nitrogen. To every 21 parts of oxygen the air contains 79 parts of nitrogen. There are always present in the air some dust, moisture, and other impurities when atmosphere is put in motion by the unequal distribution of the heat. When such unequal distribution of heat occurs the air is called wind. By exposing a large canvas surface to the wind, boats may be propelled. The farmer utilizes the wind to turn a large wheel or windmill, and thus pump his water from a well.

FIG. 54.   Aneroid Barometer.

FIG. 54. - Aneroid Barometer.