If you fill a wineglass with water and place a thick piece of paper
over it so that no air can get in, you will find that you can turn
the glass upside down without spilling a drop of water, because the
pressure of the air on the outside will keep the paper from falling
off. It is on this principle that the present pendulum is to be made.
Take a piece of cardboard larger than the mouth of the glass; pass a
cord through a small hole in the center of the card, and fasten it by
means of a knot on the under side, then carefully cover the hole with
wax, so that no air may get in.
Place your cardboard over the glass full of water, and by making a
loop in the end of the cord you can hang the glass from a hook in the
ceiling without any fear of its falling off. In order to make sure
that no air can get into the glass, it is wise to smear the rim with
tallow before laying the cardboard on.
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