THE ATMOSPHERE 115 



Air penetrating the tissues of our bodies maintains a pres- 

 sure within equal and opposite to the pressure from without. 

 We are unconscious of the existence of any such air pressure 

 until in some way an inequality is brought about in it. 



For changes in altitude on or near the earth's surface the 

 barometer falls about one inch for every 1000 feet elevation. 

 In balloon ascensions where the rise is rapid, the aeronaut 



PIG. 42. The manufacture of artificial ice. V, Valve regulating pressure; 

 A, water supply, and drip, to carry off heat from compression of the ammonia 

 gas; /, containers of distilled water to be frozen; S, brine (solution of low 

 freezing temperature); B, compression chamber; C, expansion chamber. 



sometimes bleeds at the nose and mouth because of the 

 rupture of the walls of blood vessels. This is due to the fact 

 that the gaseous pressure within the blood soon becomes 

 greater than the outer air pressure, the latter being rapidly 

 reduced because of the ascent. 



It is customary not to fill balloons full of gas before they 

 are sent up owing to the expansion of the gas as the balloon 

 rises into the less dense upper atmosphere. The heat of the 



