The Melodion, an instrument of great power, embracing five octaves, operates by means of the vibrations of metallic rods of unequal lengths, fixed at one end and free at the other.[19] A narrow and thin plate of copper is screwed to the free extremity of each rod, and at right angles to its length; and its surface is covered with a small piece of felt, impregnated with rosin. This narrow band is placed near the circumference of a revolving cylinder, and, by touching the key, it is made to descend till it touches the revolving cylinder, and gives out its sound. The sweetness and power of this instrument are unrivalled; and such is the character of its tones, that persons of a nervous temperament are often entirely overpowered by its effects. [19] See _Edinburgh Encyclopædia_, Art. SCIENCE, Curiosities in, Vol. xvii., p. 563. The vibrations of plates of metal or glass of various forms exhibit a series of the most extraordinary phenomena, which are capable of being shown by very simple means. These phenomena are displayed in an infinite variety of regular figures assumed by sand or fine lycopodium powder, strewed over the surface of the glass plate. In order to produce these figures, we must pinch or damp the plate at one or more places, and when the sand is strewed upon its surface, it is thrown into vibrations by drawing a fiddle-bow over different parts of its circumference. The method of damping or pinching plates is shown in Fig. 45. In No. 1, a square plate of glass AB, ground smooth at its edges, is pinched by the finger and thumb. In No. 2, a circular plate is held by the thumb against the top _c_ of a perpendicular rod, and damped by the fingers at two different points of its circumference. In No. 3 it is damped at three points of its circumference; _c_ and _d_ by the thumb and finger, and at _e_ by pressing it against a fixed obstacle _a b_. By means of a clamp like that at No. 4, it may be damped at a greater number of points. [Illustration: _Fig. 45._ N^o 1 N^o 2 N^o 3 N^o 4 ] [Illustration: _Fig. 46._ N^o 1 N^o 2 N^o 3 N^o 4 ] If we take a _square_ plate of glass, such as that shown in Fig. 46, No. 1, and, pinching it at its centre, draw the fiddle-bow near one of its angles, the sand will accumulate in the form of a cross, as shown in the figure, being thrown off the parts of the plate that are in a state of vibration, and settling in the nodes or parts which are at rest. If the bow is drawn across the middle of one of the edges, the sand will accumulate as in No. 2. If the plate is pinched at N, No. 3, and the bow applied at F and perpendicular to AB, the sand will arrange itself in three parallel lines, perpendicular to a fourth passing through F and N. But if the point N, where it is pinched, is a little farther from the edge than in No. 3, the parallel lines will change into curves as in No. 4.