WHAT WE KN-OW ABOUT AIR 



Altitude 



Sea level 



Mount McKinley 3.8 miles 



Mount Everest S.5 miles 



Highest point reached by 



airplane 8.3 miles 



Highest point reached by 



balloon over 14 miles 



Highest point reached by 



sounding balloon 21 miles 



Pressure 



30 inches of mercury 

 IS inches of mercury 

 10 inches of mercury 



6 inches of mercury 

 Less than 2 in. of mercury 

 Less than 1 in. of mercury 



In 1650, Otto von Guericke, mayor of the city of 

 Magdeburg and one of the early experimenters with 

 the pressure of air, built two large hemispheres, 22 

 inches in diameter. He so constructed these that they 

 could be fitted tightly together and the air exhausted 

 from the inside. When the air had been pumped from 

 within, he found that four teams of horses hitched 

 to each side of the hemispheres were unable to sep- 

 arate them. He demonstrated his experiment before 

 Emperor Ferdinand III. 



How can the pressure of air be proved and meas- 

 ured? About the year 1641, Galileo, a great Italian 

 scientist, and his pupil Torricelli, who were among 

 the first to attempt the measurement of the pressure 

 of the atmosphere, noticed that an ordi- 

 nary lift pump would not work where the 

 water in the well was more than thirty- 

 three feet below the pump. They sug- 

 gested that perhaps this water column 

 pushed back as much as the atmosphere 

 could push up. Torricelli further sug- 

 gested that it would be easier to experi- 

 ment with mercury because it is much 

 heavier, volume for volume, than water, 

 and therefore a shorter column of it would 

 weigh as much as the long column of 

 water. He constructed a tube similar to 

 the one which you used and performed 

 almost the same experiment. He found 

 that a mercury column about thirty inches 

 in length would press just about as much 

 as a column of air as deep as the atmos- 

 phere or a column of water thirty-four feet 

 in length. 



Water barometers are not generally 

 used because of their size and because of 

 other difficulties. However, several have 

 been built and used, including those made 

 by Blaise Pascal, a French scientist, Von 

 Guericke, a German, and Daniell, an Eng- 

 lish scientist. To celebrate the three hun- 

 dredth anniversary of the birth of Tor- 

 ricelli the citizens of his birthplace, 

 CURY BAROM- Faenza, Italy, erected a monument to his 

 memory in the form of a huge barometer, 



Tycos 



FIG. 8. MER- 



Taylor Instrument Company 

 FIG. 9. MONUMENT TO TORRICELLI 



make a water barometer, but one containing olive oil 

 was erected instead. A picture of this barometer 

 standing more than thirty-five feet high is shown in 

 Figure 9. 



The aneroid barometer is very different from the 

 mercury column. The word aneroid means "without 

 liquid." The heart of the aneroid barometer is the 

 little shiny box with a wavy surface which you ob- 

 served in your experiment. If you could remove it 

 from the instrument and study it carefully you would 

 discover that it is a little cylindrical chamber quite 

 hollow on the inside. The box is first constructed as 

 the diagram (Fig. 10) shows, with a metal tube ex- 

 tending from it. Afterward some of the air is removed 

 from the inside of this box and the tube is sealed. The 



ETER 



the largest in the world. The original plans were to 



FIG. 10 



box therefore has its two sides pushed in because the 

 air pressure on the outside is greater than that on the 

 inside. It then resembles the diagram shown in Figure 

 11. Now when the pressure of the air changes, the 

 shape of the box Changes also. If the pressure in- 

 creases, the sides of the box are pressed more nearly 

 together, while if it decreases, the sides spring out 

 more nearly straight. When the box is mounted prop- 

 erly this motion of its sides operates a system of lev- 

 ers, which in turn magnify the motion and pass it on 

 to the hand which moves over the scale on the dial. A 



