If, instead of moving the hand to-and fro, it be caused to describe a small circle, the ventral segments become “surfaces of revolution.” Instead of the hand, moreover, we may employ a hook turned by a whirling-table. Before you is a cord more rigid than the India-rubber tube, 25 feet long, with one of its ends attached to a freely-moving swivel fixed in the ceiling of the room. By turning the whirling-table to which the other end is attached, this cord may be divided into as many as 20 ventral segments, separated from each other by their appropriate nodes. In another arrangement a string of catgut 12 feet long, with silvered beads strung along it, is stretched horizontally between a vertical wheel and a free swivel fixed in a rigid stand. On turning the wheel, and properly regulating both the tension and the rapidity of rotation, the beaded cord may be caused to rotate as a whole, and to divide itself successively into 2, 3, 4, or 5 ventral segments. When we envelop the cord in a luminous beam, every spot of light on every bead describes a brilliant circle, and a very beautiful experiment is the result. § 4. _Mechanical Illustrations of Damping Various Points of Vibrating Cord_ The subject of _stationary waves_ was first experimentally treated by the Messrs. Weber, in their excellent researches on wave-motion. It is a subject which will well repay your attention by rendering many of the most difficult phenomena of musical strings perfectly intelligible. It will make the connection of both classes of vibrations more obvious if we vary our last experiments. Before you is a piece of India-rubber tubing, 10 or 12 feet long, stretched from _c_ to _a_, Fig. 39, and made fast to two pins at _c_ and _a_. The tube is blackened, and behind it is placed a surface of white paper, to render its motions more visible. Encircling the tube at its centre _b_ (1) by the thumb and forefinger of my left hand, and taking the middle of the lower half _b a_ of the tube in my right, I pluck it aside. Not only does the lower half swing, but the upper half also is thrown into vibration. Withdrawing the hands wholly from the tube, its two halves _a b_ and _b c_ continue to vibrate, being separated from each other by a node _b_ at the centre (2). [Illustration: FIG. 39.]