Dr M'Kendrick showed this apparatus as made by Dr R. Koenig. It consists of four small and light cylindrical resonators, made of aluminium, tuned to Ut4 = 512 vibs. p. sec., attached to the ends of four light bars forming a cross. The cross, carrying the resonators, is balanced on a fine pivot placed vertically. When the fork is sounded (by an electro-magnet between the limbs) the cross carrying the four resonators begins to rotate. The motion arises from the greater pressure at the node at the closed end in the interior of the resonator nearest the source of sound than on the outer surface of this end. If the mouth of the cylinder is directed towards the source of sound, the resonator is repelled by the greater pressure within. The next resonator is slightly attracted, but it is brought within the sphere of greater pressure mainly by the inertia of the first resonator, and so on.