Ebook A Manual of the Barometer

The curious may amuse themselves with the action of the weight of the atmosphere in the following manner: Take a glass tube of uniform bore, open at both ends; fit a cork to it, and cement a wire into the cork, which will form a piston to the tube; place the piston even with the lower end of the tube; and in that situation place the same end of the tube in mercury; hold the tube steadily and pull up the piston the mercury will follow the piston, and will fill that part of the tube Which is below the piston. By this means the weight of the atmosphere is removed from off the mercury, which is forced into the tube as far as the piston, by the weight of the atmosphere on the rest of the surface of the mercury in the basin; when the mercury in the tube balances the weight. Of the atmosphere, it remains stationary; and on pulling the piston higher, the space between it and the mercury is called a vacuum, or space void of air.