Atmospheric Pressure

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“The pressure of the atmosphere may be easily shown by the tube and piston. suppose there is an orifice to be opened or closed by the valve b, as the piston a is moved up or down in its barrel. The valve being fastened by a hinge on the upper side, on pushing the piston down, it will open by the pressure of the air against it, and the air will make its escape. But when the piston is at the bottom of the barrel, on attempting to raise it again, towards the top, the valve is closed by the force of the external air acting upon it. If, therefore, the piston be drawn up in this state, it must be against the pressure of the atmosphere, the whole weight of which, to an extent equal to the diameter of the piston, must be lifted, while there will remain a vacuum or void space below it in the tube. if the piston be only three inches in diameter, it will require the full strength of a man to draw it to the top of the barrel, and when raised, if suddenly let go, it will be forced back again by the weight of the air, and will stright the bottom with great violence.” —Comstock, 1850


J. L. Comstock A System of Natural Philosophy: Principles of Mechanics (: Pratt, Woodford, and Company, 1850) 140


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