641. What is the amount of atmospheric pressure at the earth's surface?
The pressure of the atmosphere at the earth's surface is fifteen pounds to every square inch of surface. That is to say, that the column of air, extending fifty miles over a square inch of the earth, presses upon that square inch with a weight equal to fifteen pounds.
642. Is that the weight of dry or moist air?
That is the weight of air at what is called the point of saturation, when it is fully charged with watery vapour.
643. What is the proportion of watery vapour in the atmosphere?
The proportion constantly varies. Evaporation is not a result of accident; it seems an established law that the air shall constantly absorb vapour until it has reached the maximum that it can hold. Experiments have been tried, in which dry air has been pressed upon the surface of water with great force, but no degree of pressure could prevent the formation of vapour. (See [431].)
644. What is the total amount of atmospheric pressure on the earth's surface?
The total amount of atmospheric pressure on the earth's surface, at 15 lbs. to the square inch, amounts to 12,042,604,800,000,000,000 lbs. This pressure is equal to that of a globe of lead of sixty miles in diameter.
645. What is the pressure of the atmosphere upon the human body?