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