AIR AT REST, THE ATMOSPHERE AND STATIC LIFT
Air at sea-level consists by volume of 78 per cent, nitrogen, 21 per cent, oxygen, and nearly 1 per cent, argon, together with traces of neon, helium, possibly hydrogen, and other gases.
Although the constituent gases are of different densities, the mixture is maintained practically constant up to altitudes of about 7 miles in temperate latitudes by circulation due to winds. This lower part of the atmosphere, varying in thickness from 4 miles at the poles to 9 miles at the equator, is known as the troposphere. Above it is the stratosphere, a layer where the heavier gases tend to be left at lower levels until, at great altitudes, such as 50 miles, little but helium or
hydrogen remains. Atmospheric air contains water-vapour in varying proportion, sometimes exceeding 1 per cent, by weight.
From the point of view of kinetic theory, air at a temperature of C. and at standard barometric pressure (760 mm. of mercury) may be regarded statistically as composed of discrete molecules, of mean diameter 1-5 X 10 ~ 5 mil (one-thousandth inch), to the number of 4-4 x 1011 per cu. mil. These molecules are moving rectilinearly in all directions with a mean velocity of 1470 ft. per sec., i.e. onethird faster than sound in air. They come continually into collision with one another, the length of the mean free path being 0-0023 mil.
Air at sea-level consists by volume of 78 per cent, nitrogen, 21 per cent, oxygen, and nearly 1 per cent, argon, together with traces of neon, helium, possibly hydrogen, and other gases.
Although the constituent gases are of different densities, the mixture is maintained practically constant up to altitudes of about 7 miles in temperate latitudes by circulation due to winds. This lower part of the atmosphere, varying in thickness from 4 miles at the poles to 9 miles at the equator, is known as the troposphere. Above it is the stratosphere, a layer where the heavier gases tend to be left at lower levels until, at great altitudes, such as 50 miles, little but helium or
hydrogen remains. Atmospheric air contains water-vapour in varying proportion, sometimes exceeding 1 per cent, by weight.
From the point of view of kinetic theory, air at a temperature of C. and at standard barometric pressure (760 mm. of mercury) may be regarded statistically as composed of discrete molecules, of mean diameter 1-5 X 10 ~ 5 mil (one-thousandth inch), to the number of 4-4 x 1011 per cu. mil. These molecules are moving rectilinearly in all directions with a mean velocity of 1470 ft. per sec., i.e. onethird faster than sound in air. They come continually into collision with one another, the length of the mean free path being 0-0023 mil.
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