Using the ruler, draw a line that crosses the circle at two points (A and B). Take the compass and put it on point A, and then extend the compass past the center (you'll have to guess the approximate center for this), now draw and arc. Then take the compass, keeping it open the same exact distance, and place it at point B, and draw another arc. The two arcs should cross in two places. Take a ruler and and connect the two points. This should also cross the original line AB, and that point will be the center of the cirlce.