218 PHYSIOGRAPHY 



as saying that there is more air in a cubic foot of space at sea-level 

 than in a cubic foot of space at higher levels. This means that the 

 particles of which the air is composed are nearer together at low 

 altitudes than at high altitudes. The reason why air is denser 

 below may be understood readily. If air or any other gas is put 

 under pressure, its particles are crowded together, and it is made 

 denser. At the bottom of the atmosphere the air is pressed down 

 by all the air above, and at the height of 1,000 feet the air is pressed 

 down by all the air above that level, and so on. Hence the lowest 

 air is under most pressure. This is the reason why it is densest. 



It is largely because the air is thin at high levels that mountain- 

 climbing is difficult. As the climber gets higher and higher, it 

 becomes more and more difficult to breathe. He may take in the 

 same number of cubic inches of air each time he inhales, but each 

 cubic inch contains less air the higher he goes. Furthermore, the 

 body is not used to the lessened pressure of the higher altitudes, 

 so that it causes discomfort. 



Height. How high above the sea and land does the air extend? 

 No positive answer can be given to this question, though some- 

 thing is known about it. 



1. The greatest altitude reached by any mountain-climber is 

 about four and one-half miles. At this height there was still air 

 enough to make breathing possible to a climber. This shows that 

 the air extends to heights of more than four miles and a half. 



2. Men have gone up in balloons to heights of nearly six miles. 

 In some cases the men in the balloons have become unconscious 

 at an elevation of about 29,000 feet, and in other cases oxygen 

 has been carried for breathing. Balloons without men have risen 

 ten miles. At this height the air was still dense enough so that 

 the balloon did not sink. This shows that the air extends up more 

 than ten miles. 



3. On almost any clear night "shooting stars" may be seen. 

 These shooting stars, or meteors, are small solid bodies which 

 come into the earth's atmosphere from the space outside. They 

 are very cold when they enter the earth's atmosphere, for the 

 temperature of space, outside the earth's atmosphere, is very low 

 (believed to be about 459 F.). In passing through the atmos- 



