mosphere. 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 convenien