Magic Figures.
Upon a little square of ordinary writing-paper, sketch some geometrical figure—square, rectangle, triangle, polygon, etc.—using for that purpose a pencil whose tip has been moistened with water. Float the paper, with the design upward, on the water in a basin, and fill up with water the figure you have traced. With a little care you may do this without difficulty, for the lines of moisture which form the outlines of your figure (a triangle, we will suppose) will prevent the liquid overpassing the limits thereby defined. The water thus enclosed will rise in a little heap. Now take a pin, and placing the point at any spot you please within the triangle, in such manner that the point dips into the water but does not touch the paper itself, you will see the paper begin to move horizontally in a straight line until the geometric centre of the triangle places itself exactly under the point of the pin. You can readily determine beforehand this central point, which we will call A, and holding the pin as shown in the engraving, you will find that the paper travels in the direction shown by the arrow, till A comes just under the point of the pin, when it will stop of its own accord. Repeat the experiment with a square or a rectangle, and you will find that the spot which is beneath the pin-point, when the paper comes to a standstill, is precisely the point of intersection of the two diagonals.