Modern Magic: A Practical Treatise on the Art of Conjuring

There is another form of watch-mortar, which is frequently used, though to our own taste it is very inferior to that above described. It consists of a cylindrical tin box or case, about four inches high and three in diameter, open at the top, standing on a broad flat foot. Within this fits loosely another similar cylinder, of about an inch less in depth. The upper edge of this latter is turned over all round, giving the two the appearance of being both of a piece. The whole is closed by an ornamental cardboard cover, also cylindrical. If this cover be lifted lightly--i.e., without pressure--it will come off alone; but if its sides are pressed, they will clip the turned-over edge of the upper or moveable compartment, and lift this with it. In this form of the trick the borrowed watch is placed in a little bag, and the two together deposited in the upper compartment. In the mortar proper--i.e., the space between the two compartments--is placed beforehand a similar little bag, containing the broken fragments of a watch. The cover being under some pretext put on, the upper compartment is lifted off with it, and the pounding consequently falls on the prepared fragments.