GENERAL CONCEPTION OF THE ATMOSPHERE 219 



phere they are heated by friction with the air, and when they get 

 red-hot they glow and may be seen. The height at which they 

 begin to glow has been estimated in some cases, and is found to 

 be, at a maximum, more than 100 miles above sea-level. This 

 shows that the atmosphere is much more than 100 miles high, for 

 the meteors must have come through the rare, cold, upper air a long 

 distance before becoming red-hot by friction with it. 



From these considerations it appears to be certain that the air 

 extends much more than 100 miles above the rest of the earth, 

 but how much more is unknown. Whatever its height, one-half 

 the atmosphere (by weight) lies below a plane about 3.6 miles 

 above sea-level, and three-fourths of it below a plane 6.8 miles above 

 the same level. The highest mountain is about six miles high, so 

 that nearly three-fourths of the atmosphere lies below the level 

 of its top. 



Volume and mass. Since the height of the air is not known, its 

 volume cannot be determined. 



Its mass (measured by its weight) is far less than that of the 

 solid part of the earth, or even than that of the water. It has 

 been estimated at about -^^ that of the water, and about Tnrhnnr 

 that of the rest of the earth. Its weight is about equal to that of 

 a layer of water completely covering the earth to a depth of about 

 33 feet. 



History. It is probable that the atmosphere has undergone 

 changes in mass and volume in the course of its history. It was 

 formerly supposed that the atmosphere was gradually becoming 

 less, and that it would, in time, disappear. But this belief does 

 not appear to be well founded. The atmosphere is now gaining 

 various gases from volcanic and other vents (p. 170), and probably 

 has always done so. It is probably getting gases from space also, 

 and though the contributions from this source are small now, 

 they may not always have been so. The atmosphere is losing as 

 well as gaining. Some gases, especially light ones like hydrogen, 

 probably escape the attractive control of the earth and pass off 

 into space. Other constituents of the air, especially oxygen and 

 carbon dioxide, are withdrawn from the air and locked up for long 

 periods at least, if not permanently, in the rocks. The rates both 



