ABSTRACT
The air is a gaseous substance of which the atmosphere is composed. It is transparent, perfectly elastic and highly compressible. Although very light, it has a perfectly definite weight in consequence of which it exerts pressure. The entire earth is enveloped by the airspace and may be considered as a gaseous sea at the bottom of which we all live and which extends upwards to a considerable height, which constantly diminishes in density as altitude increases. 178
