THE ATMOSPHERE. 319 



condition, its height, as inferred from the barometer, 

 would be about five miles and a half. The density of 

 the air, however, diminishes with the pressure upon it, 

 so that at the height of 11,556 feet, the atmosphere is 

 of half density ; or one volume of air, as taken at the 

 surface of the earth, is expanded into tw r o at that height. 

 Thus the weight is continually diminishing ; but this is 

 regularly opposed by the decreasing temperature, which 

 diminishes the rate of about one degree for every 352 

 feet of ascent, although in all probability it is less 

 rapid at great distances from the earth. 



It has been calculated from certain phenomena of 

 refraction, that our atmosphere must extend to about 

 forty miles from the surface of the earth. It may, in 

 a state of extreme tenuity, extend still further ; but 

 it is probable that the intense cold produced by rare- 

 faction sets limits to any extension much beyond this 

 elevation. 



The uses of the atmosphere are many. It is the 

 medium for regulating the dispersion of watery vapours 

 over the earth. If there were no atmosphere, and 

 that, as now, the equatorial climes were hot and the 

 poles cold, evaporation would be continually going on 

 at the equator, and condensation in the colder regions. 

 The sky of the tropical climes would be perpetually 

 cloudless, whilst in the temperate and arctic zones we 

 should have constant rain and snow. By having a 

 gaseous atmosphere, a more uniform state of things is 

 produced; the vapours arising from the earth become 

 intimately mixed with the air, and are borne by it 

 over large tracts of country, and only precipitated 

 when they enter some stratum much colder than that 

 which involves them. There are opposite tendencies in 

 an atmosphere of air and one of vapour. The air cir- 

 culates from the colder to the warmer parts, and the 

 vapour from the warmer to the colder regions ; and as 



