NODE, NonAt. Peril. When a siring or metallic eon], under tension. is made to vi brate. there are heard besides the principal sound secondary and shriller sounds. whieli are denomi nated overtones or harmonic sounds, and are pro dueed by a certain portion of the cord vibrating independently. Investigation shows that every vibrating string is divided into a number of seg ments vibrating independently, and that the points which separate these portions from each other are at rest. These points are known as nodal points, and their situ:1611u may be found by placing small pieces of paper on a stretched string, and causing it to vibrate. When a string vibrating to its fundamental note, that is as a unit, is touched at its middle point with a feath er, it will immediately resolve itself into seg ments of equal length vibrating twice as rapidly. The point where the string is touched with the feather being at rest would accordingly forum a nude, while the vibrating portions would be known as ventral segments. Chladni discovered that if a 'date of glass or metal be clamped at the centre and then be touched by the finger at some middle point on one of the sides, and a well resined fiddle how be drawn across the edge. par
ticles of tine dust or sand previously placed on the plate will arrange themselves in lines, show• ing that along these lines no vibration has taken place; these lines are nodal lines, and are found in most cases to group themselves together into geometrical figures. and occasionally to present the most beautiful designs. (See I FIG titEs.) The arrangement of the nodal lines de pends on the point at which the plate is touched, and on the form of the plate itself. Similarly, a column of air vibrating in an open or closed organ pipe will divide itself into portions each in a state of vibration, and separated from one another by transverse sectional portions in which the air is at rest : these latter sections are known as nodal mellow:. In the open pipe the node is at the eentre of the pipe, while in the closed pipe the node is at the end. See Acousrics.