For propelling on ball horizontally and dropping another at the same instant. Both balls reach the floor at the same time, although one reaches it after describing a parabla, while the other drops in a straight line. The spring may be adjusted so as to propel the ball with greater or less force. The maximum horizontal distance through which the ball is propelled when the apparatus rests on the top of the usual 30-inch laboratory table is about 4 feet. Mounted on a finished base casting, with 13 mm by 20 cm for attaching the apparatus to a table clamp.