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?_ Estimating the surface of man's body to be equal to _fifteen square feet_, he sustains an atmospheric pressure of 32,400 lbs., or nearly _fourteen tons and a-half_. The mere _variation of weight_, arising out of the changes in the state of the atmosphere, may amount to as much as a _ton and a-half_. [Verse: "I therefore so run, not as uncertainly; so fight I, not as one that beateth the air."--CORINTH. IX.] 646. _Why does not man feel this pressure?_ Because the diffusion of air which, _surrounding him in every direction_, and acting upon the _internal_ as well as the _external_ surfaces of his body, and probably _surrounding every atom of his frame_, establishes an equilibrium, in which every degree of pressure _counteracts and sustains itself_. 647. _What is the weight of air relative to that of water?_ A cubic foot of air weighs only 523 grains, a little more than _an ounce_; a cubic foot of water weighs _one thousand ounces_. 648. _What is the greatest height in the atmosphere which any human being has ever reached?_ M. Gay Lussac, in the year 1804, ascended to the height of 23,000 feet. 649. _What is a vacuum?_ A vacuum is a space _devoid of matter_. The term is generally applied to those instances in which air is drawn from within an air-tight vessel. 650. _Is it possible to form a perfect vacuum?_ It is probably _impossible to do so_, even with the most powerful instruments--some portion of air would remain, but in so thin a form that it would be _imperceptible_. 651. _Why does the depression of a pump-handle cause the water to flow?_ Because the putting down of the handle lifts up the piston with its _valve closed_, thereby tending to produce a _vacuum_; but _the pressure of the air_ upon the water _not contained in the pump_, forces more water up into the part where a _vacuum_ would otherwise be formed. Then, when the handle is raised, and the piston forced downwards, _the valve opens_, and the water rushes through. There is a second valve, below the piston, which closes with the downward movement, to prevent the water from _rushing back again._