that upon the awl, then the centre of gravity of both the pasteboard and the ball and string are as near as they can get to the centre of the earth; that is to say, the whole of the attracting power of the earth is, as it were, centred in a single point of the cardboard; and this point is exactly below the point of suspension. All I have to do, therefore, is to draw a line, A b, corresponding with the string, and we shall find that the centre of gravity is somewhere in that line. But where? To find that out all we have to do is to take another place for the awl (fig. 4), hang the plumb-line, and make the same experiment, and there [at the point c] is the centre of gravity—there where the two lines which I have traced cross each other; and if I take that pasteboard, and make a hole with the bradawl through it at that point, you will see that it will be supported in any position in which it may be placed. Now, knowing that, what do I do when I try to stand upon one leg? Do you not see that I push myself over to the left side, and quietly take up the right leg, and thus bring some central point in my body over this left leg. What is that point which I throw over? You
