1/ Get a circle | 2/ Get a book that's at least as big as the circle. (If you haven't got a book you can use a rectanglular piece of paper). | 3/ Put one corner of the book on the edge of the circle (See "C") Mark where the two edges of the book cross the circle (See "X" and "X") | |
4/ Remove the book and join the marks with a straight line. This will be a diameter of the circle | 5/ Do it again to get another diameter. | 6/ Where the diameters cross is the centre. |
This works thanks to the fact that the angle in a semi-circle is always a right angle. There's more of this stuff in Vicious Circles and other Savage Shapes