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When the air has been exhausted, the stopcock is turned to prevent the ingress of air; the hemispheres can then be separated only by a pull that depends on the difference between the internal and the external pressure.
170. Measurement of Atmospheric Pressure. - While the above experiments have demonstrated the existence of atmospheric pressure, they have given no accurate idea of its amount. The principle employed in finding its amount is the same as that of balancing columns used in finding the specific gravity of liquids. If a bottle is filled with water B and inverted with its mouth under the surface, as in Fig. 155, the water will remain in the bottle. The downward. pressure of water in the bottle A is counterbalanced by the downward pressure of the air upon the surface of the water in B, since the downward pressure upon the surface is transmitted into an upward pressure at the mouth of the bottle.
In order to balance the entire pressure of the atmosphere by a water column, the bottle in Fig. 155 would need to be extended into a tube about 34 ft. long; but by using a liquid heavier than water, a correspondingly shorter tube can be used.
Demonstration. Close one end of a glass tube 80 cm. long and about 6 mm. in internal diameter. Fill it nearly full with clean mercury. Close the open end with the finger, and invert it several times to remove all air bubbles clinging to the sides of the
tube. Now fill the tube full, put a finger over the open end, invert it, and place the cpen end beneath the surface of mercury in a dish. Remove the finger carefully so that no air shall get into the tube. The mercury will fall a little, and the height at which it stands will measure the atmospheric pressure (Fig. 156).
171. Atmospheric Pressure at Sea Level. The average height at which the mercury column stands, at the level of the sea, is 76 cm., and this height is independent of the diameter of the tube. If the area of the cross section of the tube is 1 sq. cm., the volume of the mercury will be 76 c.c., and since its specific gravity is 13.596, its weight will be 1033.3 g. In English measure the average height of the column is 30 in., and if its cross section is 1 sq. in., its weight will be 14.7 lb. Since this weight is the measure of the pressure of the air, we can state the following:
The average pressure of the atmosphere at sea level is 14.7 lb. per square inch, or 1033.3 g. per square centimeter. This is called a pressure of 1 atmosphere, and as it is constantly changing, it is often called in round numbers 15 lb. per square inch, or 1 kg. per square centimeter.
172. The Barometer. The experiment with the glass tube filled with mercury was first made by Torricelli in 1643, and the space above the mercury column in the tube is called a Torricellian vacuum.
The mercurial barometer consists of a glass tube about 34 in. long, filled with mercury, and inverted with its lower end constantly below the surface of mercury in a cistern. It is fixed in a vertical position with a scale C graduated along the top near the end of the mercury column, the zero of this scale being the surface B of the mercury in the cistern A at the bottom (Fig. 157).
In reading the barometer a vernier scale is generally used to secure accuracy. The vernier must be brought to the top of the convex surface of the mercury, and the eye must be on a horizontal line from the top of the column; this may be secured by placing a small vertical mirror behind the top of the column, and placing the eye so that its image and the top of the column coincide. Before the height is read, the surface of the mercury in the cistern must be brought to the fixed zero. This is done by turning the screw c which raises or lowers the mercury in the cistern until it just touches the point of a pin projecting downward from the frame of the instrument, which point is the zero of the scale.
If a liquid less dense than mercury is used, the column will be correspondingly longer, and changes in it, caused by changes in atmospheric pressure, will be correspondingly greater. The glycerin barometer has a height of about 27 ft., and a change of nearly 11 in. for every change of 1 in. in the mercury barometer.
173. The Aneroid Barometer takes its name from two Greek words meaning "without fluid." It consists of a circular box of thin metal, with corrugated sides, a cross section of which is shown in Fig. 158. One side of the box, or vacuum chamber, as it is called,
с FIG. 157