_56. The Barometer._--The modern barometer (Fig. 33), consists of a Torricellian tube properly mounted. Reading a barometer consists in accurately reading the height of the mercury column. This height varies from 75 to 76.5 cm. or 29 to 30 in. in localities not far from the sea-level. The atmospheric pressure varies because of disturbances in the atmosphere. It is found that these disturbances of the atmosphere pass across the country from west to east in a somewhat regular manner, hence a series of readings of the barometer may give reliable information of the movement of these disturbances and so assist in forecasting the weather. The weather Bureau has observations taken at the same moment at various stations over the country. These observations form the basis for the daily forecast of the weather. [Illustration: FIG. 34.--An aneroid barometer] Another form of barometer in common use is the _Aneroid Barometer_ (Fig. 34). Its essential parts are a cylindrical air-tight box with an elastic corrugated cover. Inside the box is a partial vacuum. This makes the cover very sensitive to slight changes of pressure. The motion of the top of the box is conveyed by a series of levers to an indicating hand which moves over a dial. This barometer can be made so sensitive as to indicate the change of air pressure from a table top to the floor. It is much used by travelers, explorers, surveying parties and aviators, since the mercurial barometer is inconvenient to carry. Important Topics 1. _Weight_ and _Pressure_ of air in English and metric units. How shown. Evidences. 2. Work of Galileo, Torricelli, and Perrier. 3. Barometer: construction, action, mercurial, aneroid. [Illustration: FIG. 35.--Air pressure keeps the water In the tumbler.] [Illustration: FIG. 36.--Cross-section of a modern drinking fountain.] Exercises 1. Do you think Archimedes' Principle applies to the air? Does Pascal's Law? Why? 2. Find the downward pressure of the mercury in a barometer tube if the cross-section is 1 sq. cm. and the height 75 cm. at the level of the mercury surface in contact with the air. (The density of mercury is 13.6 grams per cc.) 3. What is the weight of the air in a room if it is 10 × 8 × 4 meters? 4. What weight of air is in a room 10 × 15 × 10 ft.? 5. When smoke rises in a straight line from chimneys, is it an indication of a high or low barometric pressure? Why? 6. Why does a tumbler filled with water and inverted in a dish with its rim under water remain full? 7. If the barometer tube is inclined the mercury remains at the same horizontal level. How can this be explained? 8. When the mercurial barometer stands at 76 cm., how high would a water barometer stand? Explain. 9. Explain why it is possible for one to suck soda water through a tube?