There is another point I want in the next place to draw your attention to. I have here a quantity of shot; each of these falls separately, and each has its own gravitating power, as you perceive when I let them fall loosely on a sheet of paper. If I put them into a bottle, I collect them together as one mass; and philosophers have discovered that there is a certain point in the middle of the whole collection of shots that may be considered as the _one point_ in which all their gravitating power is centred, and that point they call the _centre of gravity_: it is not at all a bad name, and rather a short one--the centre of gravity. Now suppose I take a sheet of pasteboard, or any other thing easily dealt with, and run a bradawl through it at one corner A (fig. 3), and Mr. Anderson hold that up in his hand before us, and I then take a piece of thread and an ivory ball, and hang 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 will know at once that it is the _centre of gravity_--that point in me where the whole gravitating force of my body is centred, and which I thus bring in a line over my foot. [Illustration: Fig. 5. and Fig. 6.]