Stick two knives in a cork, on the same level, and opposite each other, so as to form a balance. In the bottom of the cork, at an equal distance, insert two pins, sufficiently deep not to bend under the weight which they will have to carry. Rest the pins on a flat ruler, slightly inclined, and give them a slight balancing movement. The weight of the apparatus will fall on the pin, A, on which the whole turns the knife placed at the side, B, will knock against the support, and will tend to bring the apparatus again into its original position as the oscillating movement continues, pin, B, will, in its turn, support the whole weight, and pin, A, will shift on to the other point, A, indicated in the figure. The walking cork will thus continue its movement till it has gone over the course assigned to it. This recreation is interesting, for it demonstrates once more that all bodies are attracted by the earth, and that, as soon as they are thrown off their balance they obey the force which constantly draws them down.
The Obstinate Cork.
Take a glass or metal tube, closed at one end, and cut a stopper in cork or india-rubber to its size so that it closes the tube hermetically, and take care that it glides in the tube without difficulty, and pierce it with a hole. On the top of this hole adapt a small piece of leather, rather larger than the hole, which you must take care to wet before proceeding with the following experiment. In order to be able to withdraw the cork when in the tube, you take the precaution to fasten in it both ends of a piece of string, as indicated in the figure.
These various objects being prepared, will serve to demonstrate once more the atmospheric pressure.
Lift the valve up, force the cork into the tube towards the middle, and when there, put the valve in its place again, and pull quickly on the string as if trying to pull the cork out. The latter will not come out, for the reason that producing a vacuum before itself, the atmospheric prepare will prevent it from coming out. But if instead you draw it gently towards you, it will offer much less resistance, because the outer air will enter through the smallest interval between the cork and the glass, and partly destroy the exterior pressure.