NODAL POINTS, LINES, AND SECTIONS. When a string or metallic cord, under strong tension, is made to vibrate, we hear, besides the principal sound, several secondary and shriller sounds; these are denominated harmonic sounds, and are produced each by a certain portion of the cord which vibrates independently. Further investigation has shown that every vibrating string is divided into a number of portions alternately vibrat ing in opposite directions, and that the points which separate these portions from each other are at rest: These points are known as nodal points, and their situation may be found by placing small pieces of paper on an extended string, and causing it to vibrate; the points from which the pieces of paper have not been displaced are the nodal points. If a plate of glass or metal be held in the hand, and a well-resined fiddle-bow be drawn across the edge, particles of fine dust, previously placed on the plate, will arrange them selves in lines, showing 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. '1 he arrangement of the
nodal lines depends on the point by which the plate is held, and on the form of the plate itself. Similarly, if a column of air m a wholly or partially closed tube be acted upon by the force of the breath applied through a hole at any point in its length, the column will divide itself into cylindrical 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 arc known as nodal sections.