ry exactly counterbalanced the
upward pressure of the air. As a mercury column 30 inches high and 1
inch square weighs 15 lbs., the air-pressure on a square inch obviously
is the same.

[Illustration: FIG. 152.--A Fortin barometer.]

FORTIN'S COLUMN BAROMETER

is a simple Torricellian tube, T, with the lower end submerged in a
little glass tank of mercury (Fig. 152). The bottom of this tank is made
of washleather. To obtain a "reading" the screw S, pressing on the
washleather, is adjusted until the mercury in the tank rises to the tip
of the little ivory point P. The reading is the figure of the scale on
the face of the case opposite which the surface of the column stands.

[Illustration: FIG. 153.]

THE WHEEL BAROMETER

also employs the mercury column (Fig. 153). The lower end of the tube is
turned up and expanded to form a tank, C. The pointer P, which travels
round a graduated dial, is mounted on a spindle carrying a pulley, over
which passes a string with a weight at each end. The heavier of the
weights rests on the top of the mercury. When the atmospheric pressure
falls, the mercury in C rises, lifting this weight, and the pointer
moves. This form of barometer is not so delicate or reliable as
Fortin's, or as the siphon barometer, which has a tube of the same shape
as the wheel instrument, but of the same diameter from end to end
except for a contraction at the bend. The reading of a siphon is the
distance between the two surfaces of the mercury.

A VERY SIMPLE BAROMETER

is made by knocking off the neck of a small bottle, filling the body
with water, and hanging it up by a string in the position shown (Fig.
154). When the atmospheric pressure falls, the water at the orifice
bulges outwards; when it rises, the water retreats till its surface is
slightly concave.

[Illustration: FIG. 154.]

THE ANEROID BAROMETER.

On account of their size and weight, and the comparative difficulty of
transporting them without derangement of the mercury column, column
barometers are not so generally used as the aneroid variety. Aneroid
means "without moisture," and in this particular connection signifies
that no liquid is used in the construction of the barometer.

Fig. 155 shows an aneroid in detail. The most noticeable feature is the
vacuum chamber, V C, a circular box which has a top and bottom of
corrugated but thin and elastic metal. Sections of the box are shown in
Figs. 156, 157. It is attached at the b