This force, the pressure of the air which you have just ascertained, supplies various experiments in its illustration.

Take a tin tube, for example, the tin holder of a penny pen, which you may procure at any stationer’s. Put a little water in it and make it boil so that the steam takes the place of the air.

When steaming furiously stop the mouth of the tube with a small cork, sealing the opening hermetically. Oil it a little, so it may glide with ease. If you cool the tube by plunging it in a basin of cold water, for example, the steam is condensed, forming a vacuum in the interior, and under the atmospheric pressure the cork will glide down. Fasten a string to the cork and you can withdraw it and begin the operation again. As the water gets hot, steam is reformed; you will see the cork come up again.

A capital way of making this cork is to stick the tube in a piece of potato, cutting out of the latter a perfectly-fitting cork.