AIR
75. The Instability of the Air. We are usually not conscious of the
air around us, but sometimes we realize that the air is heavy, while
at other times we feel the bracing effect of the atmosphere. We live
in an ocean of air as truly as fish inhabit an ocean of water. If you
have ever been at the seashore you know that the ocean is never still
for a second; sometimes the waves surge back and forth in angry fury,
at other times the waves glide gently in to the shore and the surface
is as smooth as glass; but we know that there is perpetual motion of
the water even when the ocean is in its gentlest moods. Generally our
atmosphere is quiet, and we are utterly unconscious of it; at other
times we are painfully aware of it, because of its furious winds. Then
again we are oppressed by it because of the vast quantity of vapor
which it holds in the form of fog, or mist. The atmosphere around us
is as restless and varying as is the water of the sea. The air at the
top of a high tower is very different from the air at the base of the
tower. Not only does the atmosphere vary greatly at different
altitudes, but it varies at the same place from time to time, at one
period being heavy and raw, at another being fresh and invigorating.
Winds, temperature, and humidity all have a share in determining
atmospheric conditions, and no one of these plays a small part.
76. The Character of the Air. The atmosphere which envelops us at
all times extends more than fifty miles above us, its height being far
greater than the greatest depths of the sea. This atmosphere varies
from place to place; at the sea level it is heavy, on the mountain top
less heavy, and far above the earth it is so light that it does not
contain enough oxygen to permit man to live. Figure 40 illustrates by
a pile of pillows how the pressure of the air varies from level to
level.
[Illustration: FIG. 40.--To illustrate the decrease in pressure with
height.]
Sea level is a low portion of the earth's surface, hence at sea level
there is a high column of air, and a heavy air pressure. As one passes
from sea level to mountain top a gradual but steady decrease in the
height of the air column occurs, and hence a gradual but definite
lessening of the air pressure.
[Illustration: FIG. 41.--The water in the tube is at the same level as
that in the glass.]
77. Air Pressure. If an empty tube (Fig. 41) is placed upright in
water, the water will not rise in the tube, but if the tube is put in
water and the air is then drawn out of the tube by the mouth, the
water will rise in the tube (Fig. 42). This is what happens when we
take lemonade through a straw. When the air is withdrawn from the
straw by the mouth, the pressure within the straw is reduced, and the
liquid is forced up the straw by the air pressure on the surface of
the liquid in the glass. Even the ancient Greeks and Romans knew that
water would rise in a tube when the pressure within the tube was
reduced, and hence they tried to obtain water from wells in this
fashion, but the water could never be raised higher than 34 feet. Let
us see why water could rise 34 feet and no more. If an empty pipe is
placed in a cistern of water, the water in the pipe does not rise
above the level of the water in the cistern. If, however, the pressure
in the tube is removed, the water in the tube will rise to a height of
34 feet approximately. If now the air pressure in the tube is
restored, the water in the tube sinks again to the level of that in
the cistern. The air pressing on the liquid in the cistern tends to
push some liquid up the tube, but the air pressing on the water in the
tube pushes downwards, and tends to keep the liquid from rising, and
these two pressures balance each other. When, however, the pressure
within the tube is reduced, the liquid rises because of the unbalanced
pressure which acts on the water in the cistern.
[Illustration: FIG. 42.--Water rises in the tube when the air is
withdrawn.]
[Illustration: FIG. 43.--The air supports a column of mercury 30
inches high.]
The column of water which can be raised this way is approximately 34
feet, sometimes a trifle more, sometimes a trifle less. If water were
twice as heavy, just half as high a column could be supported by the
atmosphere. Mercury is about thirteen times as heavy as water and,
therefore, the column of mercury supported by the atmosphere is about
one thirteenth as high as the column of water supported by the
atmosphere. This can easily be demonstrated. Fill a glass tube about a
yard long with mercury, close the open end with a finger, and quickly
insert the end of the inverted tube in a dish of mercury (Fig. 43).
When the finger is removed, the mercury falls somewhat, leaving an
empty space in the top of the tube. If we measure the column in the
tube, we find its height is about one thirteenth of 34 feet or 30
inches, exactly what we should expect. Since there is no air pressure
within the tube, the atmospheric pressure on the mercury in the dish
is balanced solely by the mercury within the tube, that is, by a
column of mercury 30 inches high. The shortness of the mercury column
as compared with that of water makes the mercury more convenient for
both experimental and practical purposes. (See Laboratory Manual.)
78. The Barometer. Since the pressure of the air changes from time
to time, the height of the mercury will change from day to day, and
hour to hour. When the air pressure is heavy, the mercury will tend to
be high; when the air pressure is low, the mercury will show a shorter
column; and by reading the level of the mercury one can learn the
pressure of the atmosphere. If a glass tube and dish of mercury are
attached to a board and the dish of mercury is inclosed in a case for
protection from moisture and dirt, and further if a scale of inches or
centimeters is made on the upper portion of the board, we have a
mercurial barometer (Fig. 44).
[Illustration: FIG. 44.--A simple barometer.]
If the barometer is taken to the mountain top, the column of mercury
falls gradually during the ascent, showing that as one ascends, the
pressure decreases in agreement with the statement in Section 76.
Observations similar to these were made by Torricelli as early as the
sixteenth century. Taking a barometric reading consists in measuring
the height of the mercury column.
79. A Portable Barometer. The mercury barometer is large and
inconvenient to carry from place to place, and a more portable form
has been devised, known as the aneroid barometer (Fig. 45). This form
of barometer is extremely sensitive; indeed, it is so delicate that
it shows the slight difference between the pressure at the table top
and the pressure at the floor level, whereas the mercury barometer
would indicate only a much greater variation in atmospheric pressure.
The aneroid barometers are frequently made no larger than a watch and
can be carried conveniently in the pocket, but they get out of order
easily and must be frequently readjusted. The aneroid barometer is an
air-tight box whose top is made of a thin metallic disk which bends
inward or outward according to the pressure of the atmosphere. If the
atmospheric pressure increases, the thin disk is pushed slightly
inward; if, on the other hand, the atmospheric pressure decreases, the
pressure on the metallic disk decreases and the disk is not pressed so
far inward. The motion of the disk is small, and it would be
impossible to calculate changes in atmospheric pressure from the
motion of the disk, without some mechanical device to make the slight
changes in motion perceptible.
[Illustration: FIG. 45.--Aneroid barometer.]
In order to magnify the slight changes in the position of the disk,
the thin face is connected with a system of levers, or wheels, which
multiplies the changes in motion and communicates them to a pointer
which moves around a graduated circular face. In Figure 45 the real
barometer is scarcely visible, being securely inclosed in a metal case
for protection; the principle, however, can be understood by reference
to Figure 46.
[Illustration: FIG. 46.--Principle of the aneroid barometer.]
80. The Weight of the Air. We have seen that the pressure of the
atmosphere at any point is due to the weight of the air column which
stretches from that point far up into the sky above. This weight
varies slightly from time to time and from place to place, but it is
equal to about 15 pounds to the square inch as shown by actual
measurement. It comes to us as a surprise sometimes that air actually
has weight; for example, a mass of 12 cubic feet of air at average
pressure weighs 1 pound, and the air in a large assembly hall weighs
more than 1 ton.
We are practically never conscious of this really enormous pressure of
the atmosphere, which is exerted over every inch of our bodies,
because the pressure is exerted equally over the outside and the
inside of our bodies; the cells and tissues of our bodies containing
gases under atmospheric pressure. If, however, the finger is placed
over the open end of a tube and the air is sucked out of the tube by
the mouth, the flesh of the finger bulges into the tube because the
pressure within the finger is no longer equalized by the usual
atmospheric pressure (Fig. 47).
[Illustration: FIG. 47.--The flesh bulges out.]
Aëronauts have never ascended much higher than 7 miles; at that height
the barometer stands at 7 inches instead of at 30 inches, and the
internal pressure in cells and tissues is not balanced by an equal
external pressure. The unequalized internal pressure forces the blood
to the surface of the body and causes rupture of blood vessels and
other physical difficulties.
81. Use of the Barometer. Changes in air pressure are very closely
connected with changes in the weather. The barometer does not directly
foretell the weather, but a low or falling pressure, accompanied by a
simultaneous fall of the mercury, usually precedes foul weather, while
a rising pressure, accompanied by a simultaneous rise in the mercury,
usually precedes fair weather. The barometer is not an infallible
prophet, but it is of great assistance in predicting the general trend
of the weather. There are certain changes in the barometer which
follow no known laws, and which allow of no safe predictions, but on
the other hand, general future conditions for a few days ahead can be
fairly accurately determined. Figure 48 shows a barograph or
self-registering barometer which automatically registers air pressure.
[Illustration: FIG. 48.--Barograph.]
Seaport towns in particular, but all cities, large or small, and
villages too, are on request notified by the United States Weather
Bureau ten hours or more in advance, of probable weather conditions,
and in this way precautions are taken which annually save millions of
dollars and hundreds of lives.
I recollect a summer spent on a New Hampshire farm, and know that an
old farmer started his farm hands haying by moonlight at two o'clock
in the morning, because the Special Farmer's Weather Forecast of the
preceding evening had predicted rain for the following day. His
reliance on the weather report was not misplaced, since the storm came
with full force at noon. Sailing vessels, yachts, and fishing dories
remain within reach of port if the barometer foretells storms.
[Illustration: FIG. 49.--Isotherms.]
82. Isobaric and Isothermal Lines. If a line were drawn through all
points on the surface of the earth having an equal barometric pressure
at the same time, such a line would be called an isobar. For example,
if the height of barometers in different localities is observed at
exactly the same time, and if all the cities and towns which have the
same pressure are connected by a line, the curved lines will be called
isobars. By the aid of these lines the barometric conditions over a
large area can be studied. The Weather Bureau at Washington relies
greatly on these isobars for statements concerning local and distant
weather forecasts, any shift in isobaric lines showing change in
atmospheric pressure.
If a line is drawn through all points on the surface of the earth
having the same _temperature_ at the same instant, such a line is
called an isotherm (Fig. 49).
83. Weather Maps. Scattered over the United States are about 125
Government Weather Stations, at each of which three times a day, at
the same instant, accurate observations of the weather are made. These
observations, which consist of the reading of barometer and
thermometer, the determination of the velocity and direction of the
wind, the determination of the humidity and of the amount of rain or
snow, are telegraphed to the chief weather official at Washington.
From the reports of wind storms, excessive rainfall, hot waves,
clearing weather, etc., and their rate of travel, the chief officials
predict where the storms, etc., will be at a definite future time. In
the United States, the _general_ movement of weather conditions, as
indicated by the barometer, is from west to east, and if a certain
weather condition prevails in the west, it is probable that it will
advance eastward, although with decided modifications. So many
influences modify atmospheric conditions that unfailing predictions
are impossible, but the Weather Bureau predictions prove true in about
eight cases out of ten.
The reports made out at Washington are telegraphed on request to
cities in this country, and are frequently published in the daily
papers, along with the forecast of the local office. A careful study
of these reports enables one to forecast to some extent the probable
weather conditions of the day.
The first impression of a weather map (Fig. 50) with its various lines
and signals is apt to be one of confusion, and the temptation comes to
abandon the task of finding an underlying plan of the weather. If one
will bear in mind a few simple rules, the complexity of the weather
map will disappear and a glance at the map will give one information
concerning general weather conditions just as a glance at the
thermometer in the morning will give some indication of the probable
temperature of the day. (See Laboratory Manual.)
[Illustration: FIG. 50. weather Map]
On the weather map solid lines represent isobars and dotted lines
represent isotherms. The direction of the wind at any point is
indicated by an arrow which flies with the wind; and the state of the
weather--clear, partly cloudy, cloudy, rain, snow, etc.--is indicated
by symbols.
84. Components of the Air. The best known constituent of the air is
oxygen, already familiar to us as the feeder of the fire without and
within the body. Almost one fifth of the air which envelops us is made
up of the life-giving oxygen. This supply of oxygen in the air is
constantly being used up by breathing animals and glowing fires, and
unless there were some constant source of additional supply, the
quantity of oxygen in the air would soon become insufficient to
support animal life. The unfailing constant source of atmospheric
oxygen is plant life (Section 48). The leaves of plants absorb carbon
dioxide from the air, and break it up into oxygen and carbon. The
plant makes use of the carbon but it rejects the oxygen, which passes
back into the atmosphere through the pores of the leaves.
Although oxygen constitutes only one fifth of the atmosphere, it is
one of the most abundant and widely scattered of all substances.
Almost the whole earth, whether it be rich loam, barren clay, or
granite boulder, contains oxygen in some form or other; that is, in
combination with other substances. But nowhere, except in the air
around us, do we find oxygen free and uncombined with other
substances.
A less familiar but more abundant constituent of the atmosphere is the
nitrogen. Almost four fifths of the air around us is made up of
nitrogen. If the atmosphere were composed of oxygen alone, the merest
flicker of a match would set the whole world ablaze. The fact that the
oxygen of the air is diluted as it were with so large a proportion of
nitrogen, prevents fires from sweeping over the world and destroying
everything in their path. Nitrogen does not support combustion, and a
burning match placed in a corked bottle goes out as soon as it has
used up the oxygen in the bottle. The nitrogen in the bottle, not only
does not assist the burning of the match, but it acts as a damper to
the burning.
Free nitrogen, like oxygen, is a colorless, odorless gas. It is not
poisonous; but one would die if surrounded by nitrogen alone, just as
one would die if surrounded by water. The vast supply of nitrogen in
the atmosphere would be useless if the smaller amount of oxygen were
not present to keep the body alive. Nitrogen is so important a factor
in daily life that an entire chapter will be devoted to it later.
Another constituent of the air with which we are familiar is carbon
dioxide. In pure air, carbon dioxide is present in very small
proportion, being continually taken from the air by plants in the
manufacture of their food.
Various other substances are present in the air in very minute
proportions, but of all the substances in the air, oxygen, nitrogen,
and carbon dioxide are the most important.