The reason why : $b a careful collection of many hundreds of reasons for things which, though generally believed, are imperfectly understood. A book of condensed scientific knowledge for the million
John Stuart Mill · en
638. _What is the atmosphere?_
The _atmosphere_ is the transparent and elastic body of mixed gases
and vapours which envelopes our globe, and which derives its name from
Greek words, signifying _sphere_ of _vapour_.
639. _To what height does the atmosphere extend?_
It is estimated to extend to from _forty to fifty miles_ above the
surface of the earth.
640. _Why is it supposed that the atmosphere does not extend beyond
that height?_
Because it is found, by experiment and observation, that the air
becomes _less dense_ in proportion to its altitude from the earth's
surface. The gradual decrease of atmospheric density observed in
ascending a mountain, or in a balloon, supplies sufficient data to
enable us to calculate the height at which the atmosphere would
probably _altogether cease_.
At an altitude of 18,000 feet the air is indicated by the
barometer to be only _half as dense_ as at the surface of
the earth. And as the densities of the atmosphere decrease
in a geometrical progression, the density will be reduced to
_one-fourth_ at the height of 36,000 feet; and to _one-eighth_
at 54,000 feet. The effects of the decreasing density of the
atmosphere are, that the _intensity of light and sound are
diminished, and the temperature is lowered_. Persons who have
reached a very high elevation, state that the sky above them began
to assume the appearance of darkness; and there can be no doubt
that, if it were possible to reach an altitude of some fifty to
sixty miles, there would be _perfect blackness although the sun's
rays might be pouring through the darkened space_, to illuminate
the atmosphere. Upon the summit of Mont Blanc, the report of a
pistol at a short distance can _scarcely be heard_. When Gay
Lussac reached the height of 23,000 feet, he breathed with great
pain and difficulty, and felt distressing sensations in his ears,
as though they were _about to burst_. Upon the high table-lands of
Peru, the lips of Dr. Ischudi cracked and burst; and blood flowed
from his eyelids.
[Verse: "For he looketh to the ends of the earth, and seeth under
the whole heaven; To make the weight for the winds."--JOB XXVIII.]
641. _What is the amount of atmospheric pressure at the earth's
surface?_
The pressure of the atmosphere at the earth's surface is _fifteen
pounds_ to every square inch of surface. That is to say, that the
column of air, extending fifty miles over a square inch of the earth,
presses upon that square inch with a weight equal to _fifteen pounds_.
642. _Is that the weight of dry or moist air?_
That is the weight of air at what is called the _point of saturation_,
when it is fully charged with _watery vapour_.
643. _What is the proportion of watery vapour in the atmosphere?_