74 



METEOROLOGY. 



ly one half of the mass of the atmosphere lies within 2^ or 3 miles of 

 the earth's surface. But as the density from that point upwards is much 

 less than that below, and is continually decreasing, the remaining half of 

 the atmosphere must occupy vastly more space. By taking into consi- 

 deration the fact, that this decreasing density of the air at a considerable 

 elevation, in connection with the great increase of cold which there pre- 

 vails, must diminish its elasticity more rapidly than the force of the 

 earth's attraction decreases, we are brought to the conclusion that the at- 

 mosphere is confined to within about 40 miles of the surface of the 

 earth. The coercive force, or that which tends to bring it all down to 

 the suiface of the earth, is the earth's attraction — the expansive force, or 

 that which resists the action of gravity, is the elasticity of the air. At 

 a height of about 40 miles they are equal to each other. Gravity will 

 not permit an expansion, nor elasticity a contraction, unless within the 

 narrow limits within which each is altered by other forces. From cal- 

 culations, also, based upon the phenomena of refraction, the atmosphere 

 is inferred to extend no further than about 45 miles from the earth. 



2. The conqwsitlon of the atmosphere . By careful analysis every 

 five cubic inches of pure and dry air are found to consist nearly of four 

 of nitrogen and one of oxygen gas. But the air is never perfectly j9«re. 

 It contains variable quantities of other gases, of which carbonic acid is 

 the most considerable, amounting sometimes to 62 parts in 100000 of 

 air, and never less than 37. But though this gas exists at all known 

 heights, its amount in proportion to the rest of the atmosphere is too 

 small to influence in a sensible manner the phenomena of meteorology. 



Neither is the air ever perfectly dry. It always contains variable 

 quantities of moisture, which plays a most important part in all atmos- 

 pheric changes. Indeed the changes dependent directly or indirectly 

 upon the moisture existing in the atmosphere, constitute the subject 

 which we have in hand. 



3. The forms under which moisture exists in the atmosphere. Mois- 

 ture exists in the atmosphere under three forms. Fii'st, it exists as in- 

 visible vapor. Any one can satisfy himself that this is the fact, by fill- 

 ing a tumbler or tin-cup with ice-cold water during summer, when he 

 will soon see its outer surface covered with large drops of water resem- 

 bling dew, or with a freezing mixture during winter, when he will soon 

 see the vessel covered with frost. Secondly., moisture exists as visible 

 vapor. The passage from the state of invisible to visible vapor is beau- 

 tifully seen by watching the steam of boiling water as it issues from the 

 spout of a tea-kettle. At the mouth of the spout and to the distance of 

 about an inch from it, nothing can be seen, because the vapor is there 



