When the wheel is rapidly revolved, the fluid is driven out with considerable force through the opening at the rim, while a partial vacuum is produced at the axle causing a rapid flow into the device at this point. This is the principle of the action of the _vacuum cleaner_. Fig. 53 is a section of a vacuum sweeper showing the revolving wheel and the current of air passing into the wheel at the lower side and out of the rim of the case at the rear. _Centrifugal water pumps_ work on the same principle and furnish a continuous flow of water, often large in volume and at considerable pressure. Important Topics 1. Air pump. 2. Condensing pump. 3. Lift and force pumps. 4. Siphon. 5. Cartesian diver. 6. Hydraulic ram. 7. Balloon. 8. Air brake. 9. Gas meter. 10. Vacuum cleaner. Exercises 1. Explain why smoke settles to the ground before storms. 2. Why does the water rise in the suction pipe of a pump? 3. Why is it easier to float in water when the lungs are filled with air than when they are not filled? 4. Why is it easier to swim in salt water than in fresh water? 5. How are submarines made to sink? to rise to the surface? 6. How can a fish rise or sink in water? 7. Explain why a life preserver made of cork will enable a person to float. 8. Hold the open hand out flat with the fingers together. Place underneath the fingers a piece of paper. Blow between the first and second fingers against the paper. As long as you blow hard the paper will not fall but will stick to the hand. Explain. 9. Why does pressing the bulb of an atomizer force out the liquid in a fine spray? 10. Why is air that contains a large amount of water vapor lighter than air that only contains a small amount? 11. How are heights above sea-level ascertained by a barometer? 12. Oil floats on water but sinks in alcohol. Explain. 13. In a balloon the lower end is often open to the air. Why does not the gas escape and prevent the balloon from rising? 14. How long will a balloon continue to rise? 15. If the pressure against the 8-in. piston of an air brake is 70 lbs. per square inch, how much force does the piston exert? 16. The capacity of a balloon is 40,000 cu. ft. The weight of the balloon, car, etc., is 600 lbs.; specific gravity of the gas used is 0.46 that of the air. Find how much weight the balloon can carry. 17. The so-called Magdeburg hemispheres were invented by Otto von Guericke of Magdeburg, Germany. When the hemispheres (see Fig. 54) are placed in contact and the air exhausted it is found very difficult to pull them apart. Explain. 18. Von Guericke's hemispheres had an inside diameter of 22 in. What force would be required to pull them apart if all the air were exhausted from them? (Find the atmospheric force on a circle, 22 in. in diameter.)