20 GRAVITATION. 



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 



