Modern Magic: A Practical Treatise on the Art of Conjuring

There is a further appliance in connection with the box in question, which, however, may be used with or without it, as may best suit the trick in hand. This is a loose slab, a, of the same wood of which the interior of the box is made, of the thickness of cardboard, and of such a size as to fit closely, though not tightly, in either half of the box. When so placed, it has the appearance of the inside top or bottom of the box. When the box is closed in such manner that the part in which this slab is placed is uppermost, the slab falls into the lower portion, thus forming a false bottom on whichever side happens to be undermost. If a card (say the ace of hearts) be secretly placed in either side of the box, and this slab placed on it, the box will appear empty. If now another card (say the knave of spades) be openly placed in either side, and the box closed in such manner that the portion containing the false bottom is undermost, no change will take place; but if, either in closing the box or subsequently, it is so placed that the side containing the false bottom becomes uppermost, the false bottom will at once drop into the opposite division, and on re-opening the box the ace of hearts will be revealed, and the knave of spades will in its turn be concealed. The effect to the spectators is as if the knave of spades had changed into the ace of hearts.