48 



GENERAL SCIENCE 



atmospheric pressure. As people get older, the blood pressure in- 

 creases, sometimes as much as 3 or 4 pounds. Insurance companies 



as well as physicians often determine 

 the probable length of a person's life 

 by the blood pressure. If a person 

 has high blood pressure at an early 

 age, it usually indicates ill health 

 and short life. 



Making the Barometer. Fill a 

 glass tube about 3 

 feet long with mer- 

 cury and invert it 

 in a small dish of 

 mercury. The mer- 

 cury will drop until 

 it stands at about 

 30 inches at sea 

 level, which shows 

 that the air pres- 

 sure is great enough 

 to make the mer- 

 cury rise, but it is 

 impossible for the 



FIG. 38. How much greater is 

 the blood pressure than the 

 atmospheric pressure? Why 

 is this necessary? 



FIG. 39. Barom- 

 eter. 



air to make the mercury rise to fill the vacuum at the top. 



Air Pressure Varies at different heights: It has been esti- 

 mated that the atmosphere extends above the earth about two hun- 

 dred miles, possibly more. At the bottom of this ocean we humans 

 live and crawl about, ordinarily on the flat lower levels at the very 

 bottom. Sometimes, after much toil, we climb the little ridges and 

 mounds called mountains, little when compared with the depth of 

 the atmosphere, though not very small when compared with our- 

 selves. The highest of the high mountain peaks are well to the 

 bottom of this ocean of air. This air is constantly growing rarer 

 the higher we get above the earth's surface. The greater part of 

 the weight of the atmosphere is within three miles of the earth. 



When airmen ascend too high, the blood comes out through the thin 

 membranes of the nose and eyes, as the inside pressure is too great. 



