Atmospheric pressure is the weight that the atmosphere exerts upon us. The measured value of the atmospheric pressure fluctuates slightly in response to moving weather systems, and it drops as one ascends to higher elevations, since less air is overhead to exert a downward force. The mean atmospheric pressure at sea level is given in a number of different unit systems as 1013.25 millibars, 29.92 inches of mercury, and 14.7 lbs per square inch. The last unit makes plain the fact the pressure is defined as weight (force) per area.

Aristotle, whose teachings sometimes inhibited the advancement of science, was right on target in his belief that the atmosphere surrounding the Earth had weight. Moreover, he believed that as air density decreased, it would be possible for an object to move faster. However, he did not believe in the concept of a vacuum because the absence of an atmosphere...