ATMOSPHERIC PRESSURE. 157 



taken as tl%e type of aeriform bodies. Whatever 

 mechanical properties are shown as belonging to air may 

 be understood as belonging to all gases. 



271. The Aerial Ocean. Air is chiefly a mixture 

 of two gases, oxygen and nitrogen, in the proportions of 

 one to four by volume. It is believed that the atmosphere 

 at its upper limit presents a definite surface like that of 

 the sea : that disturbing causes produce waves there just as 

 they do on the sea, but that, by reason of greater mobility 

 and other causes, the waves on the surface of this aerial 

 ocean are much larger than any ever seen on the surface 

 of the liquid ocean. The depth of this aerial ocean has 

 been variously estimated at from fifty to two hundred miles. 



2*72. Weight of Air. Being a form of matter, air 

 has weight. This may be shown by experiment. A hol- 

 low globe of glass or metal, having a capacity of several 

 liters and provided with a stop-cock, is carefully weighed 

 on a delicate balance. The air is then removed from the 

 globe by an air-pump, the stop-cock closed, and the empty 

 globe weighed carefully. The second weight will be less 

 than the first, the difference between the two being the 

 weight of the air removed. Under ordinary conditions a 

 cubic inch of air weighs about 0.31 grains ; a liter of air 

 weighs about 1/293 g., being thus about -^ as heavy as 

 water. (See Appendix G.) 



273. Atmospheric Pressure. Having weight, 

 such a quantity of air must exert a great pressure upon 

 the surface -of the earth and all bodies found there. This 

 atmospheric pressure necessarily decreases as we ascend 

 from the earth's surface. For any surface, at any ele- 

 vation, the upward, downward, or lateral pressure may be 



