Weather Study 863 



or in inches of mercury, the weight of which would counterbalance the 

 weight of the vapor in the air. The conditions present in a volume of 

 saturated air at a temperature of 3 2 ° may be expressed as follows : Relative 

 humidity 100%; dewpoint32°; absolute humidity 2. 11 grains per cu. ft. 

 or .18 inch. 



Pressure of Atmosphere 

 Although the atmosphere is composed of these various gases, it acts in 

 all respects like a simple, single gas. It is very elastic, easily compressed, 

 expands when heated and contracts when cooled. It is acted upon by 

 gravity and, therefore, has weight and exerts pressure, which at sea level 

 amount to about 14.7 pounds on each square inch of the surface. Because 

 it is compressible and has weight, it is more dense at the surface than at any 

 elevation above the surface, and as we ascend in the atmosphere the weight 

 or pressure decreases in proportion to the weight of that part of the atmos- 

 phere left below. The weight or pressure of the atmosphere is measured 

 by means of a barometer and is expressed in terms of inches of mercury. 

 The normal atmosphere at sea level will sustain a column of mercury 

 about thirty inches high, and we therefore say that the normal pressure 

 of the atmosphere is thirty inches. (See Lessons on air pressure and the 

 barometer.) 



The Height of the Atmosphere 



The air that surrounds the earth is called its 

 atmosphere, but it is a rather curious fact that the 

 earth has really ten atmospheres and may have 

 others not yet discovered. 



The air near the surface is a mixture of eight 



different gases, and each individual gas arranges 



itself so as to form an atmosphere just as it would 



if no other gases were present. Thus, the earth is 



surrounded by an atmosphere of oxygen, an 



atmosphere of nitrogen, one of carbon dioxide, 



Snow crystal. one of water vapor, one each of argon, krypton. 



Photomicrograph by ucon, and xenou, while hydrogen and helium 



w. A. Bentiey. are believed to exist at great elevations above the 



earth's surface. 



These gases are kept from flying off into space by the force of gravity, 



just as a piece of iron, stone, or a building is held fast to the earth by the 



same force. Gravity acts with greater force on some things than on others. 



For example, a piece of iron is pulled down by gravity with greater force 



than is a piece of wood of the same size ; likewise, a piece of lead is pulled 



down with greater force than a piece of iron. We, therefore, say that iron is 



heavier than wood and that lead is heavier than iron, simply because gravity 



acts with greater force on the one than on the other. The weight of gases 



differ just as the weight of different solids, such as lead, wood or iron differ. 



For instance, nitrogen is 14 and oxygen 16 times heavier than hydrogen. 



Gases having the least weight extend upward the farthest, because the 

 lighter the gas the greater its expansive force. Every boy who rides a 

 bicycle takes advantage of the expansive force of air when he pumps his 

 tires. The air is compressed by the pump into the tube and the expansive 

 force exerted by the air in trying to expand makes the tire "stand up." If 

 it requires 10 pounds pressure to compress the gas into the tube, the expan- 

 sive force will be just ten pounds. 



