Page:Handbook of Meteorology.djvu/15

on the other hand, increases with altitude, and they probably form the outer layer of the atmosphere.^[1] Because of their lightness it is not unlikely that hydrogen and helium are gradually escaping from the earth.

Constituents	Parts in one million of air
Nitrogen	771,200
Oxygen	206,600
Argon group (approximately)	7,900
Water-vapor	13.953
Carbon dioxide	336
Ozone	12
Nitric and nitrous oxides	8
Ammonia	1
Dust, hydrogen, helium	1(?)

Depth of the Atmosphere.—Observations on the twilight arch indicate that at a height of 40 miles above sea level the air has a density sufficient to refract, reflect, and diffract light. A measurement of the parallax of a meteor seen by two observers

Constituents	Per cent
Nitrogen	78.03
Oxygen	20.99
Argon	0.94
Carbon dioxide	0.03
Hydrogen	0.01
Neon	0.0012
Helium	0.0004

at different stations indicates the existence of air at a height of 200 miles. Actual measurements, however, have not extended much higher than 20 miles, the height to which sounding balloons have reached.

At an altitude varying approximately from 6 to 7 miles, according to latitude and also according to the season, a plane of contact occurs which apparently separates an upper from a

1. The foregoing are on the authority of W. J. Humphreys.