Because a column of quicksilver thirty inches high, weighs exactly the same as a column of air fifty miles high, or a column of water thirty-four feet high.
How can this be shown?
Take a glass tube about thirty-three inches long, cork up one end, and fill the tube with quicksilver; then put the open end of the tube beneath the surface of some quicksilver, in a basin, and the quicksilver in the tube will fall to the height of thirty inches, as is shown in [Fig. 13].
Fig. 13.
If the cork be removed from the glass tube, what will become of the quicksilver?
It will immediately sink into the basin below.
Why will the quicksilver sink into the basin?
Because the air will then press upon the quicksilver in the tube, the same as it does on that in the basin; hence, all the quicksilver will fall to the same level.
Does the weight of the air vary at different times?