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.