SOME CHARACTERISTICS OF THE AIR 5 



By attaching a barometer (sect. 5) to the air pump it is pos- 

 sible to find out what fraction of the air was left in the vessel, 

 and this enables us to tell exactly what relation the amount 

 of air removed by the pump bears to the whole amount of air 

 in the flask. By this means we may ascertain the necessary 

 corrections and discover the true weight of air. This has been 



FIG. 6. An aneroid barometer (barograph) 



This instrument automatically records the air pressure. A, metal boxes ; B, 



pointer, at the tip of which is C, the pen ; D, paper-covered cylinder which is 



rotated by a clock contained within it. As the pressure changes, the pen moves 



up or down, thus tracing its course upon the rotating cylinder of paper 



done many times. The actual weight of air varies from time 

 to time, because of changes in temperature, moisture, and 

 pressure, but at 60 F. it is about 533 grains, or 34.5 grams, 

 per cubic foot. 



4. Air pressure. If air has weight, it ought to press down 

 upon the surface of the earth just as other objects do when 

 they rest upon the earth, and it ought to rest upon us and 

 other objects on the earth's surface with considerable weight. 



