10 



THE OCEAN OF AIR 



Siphons find many uses in modern life. Aquariums, 

 wash tubs, barrels, and other containers that are diffi- 

 cult to move may be emptied by them. They are used 

 in the operation of the trap beneath sinks and wash 

 bowls and also in the operation of toilet bowls. When 

 New York City built its great water supply it was 

 necessary to get the water from 

 the west to the east side of the 

 Hudson River. After studying 

 the situation the engineers de- 

 cided that a huge siphon tunnel 



A j^=(-f><j upside down running far below 



the river would solve the prob- 

 lem, and so they tunneled down 

 and across the river and then 

 up toward the surface on the 

 other side, hundreds of feet 

 through solid rock, to make one 

 of the largest siphons in the 

 world. The city of Los Angeles 

 found that its water supply 

 must be taken over a mountain, 

 and so a great siphon was used 

 to accomplish this feat. 



What is a siphon and how 

 does it work? In the experi- 

 ment which was performed 

 FIG. 21. SIPHON with the si-phon you learned 

 that liquids may be moved from 



one level to a lower one only, and that the siphon tube 

 must first be filled with liquid. The reason for this is 

 interesting. If the glasses in Figure 16 were at the 

 same level, no water would flow because the liquid in 

 each tube of the siphon would weigh the same and 

 therefore they would counterbalance each other. In 

 the diagram (Fig. 21), glass C has been lowered until 

 the long arm BC of the siphon is about three times as 

 long as the short arm AB. Since the weight of the 

 water in tube BC is much greater than that of the water 

 in tube AB, the flow starts toward C. Air pressure push- 

 ing down on the water in glass A will push more 

 water up into the tube, and the action continues until 

 the water reaches the end of the tube. It is interesting 

 to note that several different types of self-starting 

 siphons can be made to work very well. 



What are some of the uses of the vacuum? Many 

 years ago Galileo and his associates offered the theory, 

 "Nature dislikes a vacuum," to explain the fact that 

 water rushes into the cylinder of a lift pump from a 

 well. In our modern life we seem to have found so 

 many uses for the vacuum that we do not worry about 

 whether nature likes it or not. A study of Figure 22 

 will show you how common it is in our everyday 

 experience. Can you add other uses to those pictured? 

 A vacuum is simply a space from which air and 



other substances have been removed. The pressure in 

 a vacuum space is decreased as more and more of the 

 air is removed. If a barometer tube were placed in 

 such a space it would gradually fall as the pumping 

 is continued. Recall that when the pressure is 15 

 pounds per square inch, a mercury column stands 30 

 inches high. Vacuums can be produced in which the 

 mercury column would be less than the hundredth 

 part of an inch in height. 



High vacuums are important in many branches of 

 science and have become increasingly important with 



FIG. 22. USES OF THE VACUUM 



the development of radio and X-ray tubes. High vac- 

 uums are secured by means of different types of pumps 

 that are made to remove practically all the air from a 

 tube or bulb. 



Variations in air pressure are used in the vacuum 

 sweeper. It is commorlly believed that a vacuum 

 sweeper operates through the suction of a vacuum. 

 While it is true that the whirling pump fan in a vac- 

 uum cleaner produces a partial vacuum in the nozzle 

 or part which touches the carpet, the real force which 

 gets the dust and dirt out of the meshes of the rug 

 is furnished by the pressure of the air. When the fan 

 starts to turn, the pressure back of it is increased 

 because air is forced or pushed by it into the bag. That 

 the pressure in the bag is increased may be seen by 

 the fact that the bag fills out at once. The pressure 

 just in front of the fan is considerably decreased, and 

 as the outside air under greater pressure rushes in 

 through the nozzle, it takes dust and dirt particles 

 with it. 



Atomizers and sprays make use of air pressure. 

 Perfume is often applied with an atomizer, and fre- 

 quently sore throat is treated with a spray. Each of 

 these devices operates with air pressure. When the 

 bulb is pressed, a blast of air is blown past the small 

 pipe which dips into the liquid in the glass con- 

 tainer. This reduces the pressure in the pipe, and the 

 greater pressure of air over the liquid pushes some 

 of it up into the blast where it is atomized or broken 

 up into a spray. 



