Aim the launcher directly at the monkey and fire. It is set up so that the circuit is shorted the moment the ball is fired, causing the monkey will fall at the same time as the ball is fired. Despite the fact that the monkey is traveling in a straight line while the ball is following a parabolic trajectory, they will collide (provided you have offered satisfactory and sufficient burnt offerings to the physics gods), since the vertical and horizontal components of velocity are independent of each other. It may help to use a laser pointer to align the launcher (besides, then you can tell the class that your monkey-hunting cannon has a laser sight!).