382 



AIR 



pressure of the atmosphere, which is exerted over every inch 

 of our bodies, because the pressure is exerted equally over 

 the outside and the inside of our bodies ; the cells and tissues 

 of our bodies containing gases under atmospheric pressure. 

 If, however, a finger is placed over the open end of a tube 



and the air is sucked out of the 

 tube by the mouth, the flesh of 

 the finger bulges into the tube 

 because the pressure within the 

 finger is no longer equalized by 

 the usual atmospheric pressure 

 (Fig. 252). 



Aeronauts have never ascended 

 much higher than 7 miles. At 



that height the barometer stands 

 FIG. 252. -The flesh bulges out. ^ ? inches instead of ^ ^ inches> 



and the internal pressure in cells and tissues is not balanced 

 by an equal external pressure. The unequalized internal pres- 

 sure forces the blood to the surface of the body and causes 

 rupture of blood vessels and other physical difficulties. 



Use of the barometer. Changes in air pressure are very 

 closely connected with changes in the weather. The barom- 

 eter does not directly foretell the weather, but a low or fall- 

 ing pressure, indicated by a fall of the mercury, usually pre- 

 cedes foul weather, while a rising pressure, indicated by a 

 rise in the mercury, usually precedes fair weather. The ba- 

 rometer is not an infallible prophet, but it is of great assistance 

 n predicting the general trend of the weather. Figure 253 

 shows a barograph or self-registering barometer which auto- 

 matically records air pressure. 



Seaport towns in particular, but all cities and villages, 

 are on request notified by the United States Weather Bureau, 

 ten hours or more in advance, of probable weather con- 



