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.
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.