Transportation in the Air 391 



rescue to the ingenuity of one of them, who pulled the valve 

 rope with his teeth. A French aeronaut later reached the 

 same altitude, but he carried with him a tank of oxygen to 

 supply air. These discoveries of the character of the atmosphere 

 at high altitudes led to farther investigations by the aid of 

 exploring balloons carrying no human passengers. 



The exploring balloon is equipped with delicate recording 

 instruments which respond to differences in temperature, and 

 pressure or amount of atmosphere. The quantity of hydrogen 

 in such balloons is such that its expansion will burst the bag at 

 a given altitude. Why does the gas expand as the balloon 

 ascends? The instruments are attached to a parachute which 

 saves them from destruction. As they may be carried by air 

 currents miles away from the starting place, they are tagged with 

 the address and directions for their return; when the finder 

 returns them he is rewarded for his trouble. 



The upper air its pressure and amount. The Italians have 

 led in such researches, one of their exploring balloons ascending 

 to the height of 23 miles. The record of the barometer shows 

 the pressure of the air at different altitudes. Mountain climbers 

 had reported that there is hardly enough air at the height of 

 20,000 to 24,500 feet to sustain life. At about 19,000 feet 

 practically only one half of the total amount of air is available. 

 The barometric record obtained by the Italians' balloon verified 

 these observations. At the height of 7 miles, of the air is 

 left below the balloon ; at 23 miles' height, ffj is left below, 

 on ly -giro remaining above. Since the thinning out of air from 

 sea level to the highest investigated altitude is very gradual, 

 there is a high degree of probability that this -^ part is so 

 expanded and diffused that it exists in traces a hundred miles 

 higher. Certain observations support this belief. Meteors 

 begin to glow a hundred miles distant from the earth's surface ; 

 the heat causing the glow being perhaps due to friction with the 

 air through which they move. The after-glow of sunset and the 

 aurora indicate the presence of air at such altitude ; indeed the 



