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290                    HAND-BOOK OF ACOUSTICS.
Am.—See pp. 124 and 125. The lycopodium powder being so light is entangled by the little whirlwinds of air produced by the vibrations of the plate; it cannot escape from these tiny whirlwinds, but the heavier sand particles are readily driven through them.
5. Find the form of the sand figures produced on a square plate so bowed that its vibrations, so far as they are parallel to one edge are taking place with two nodal lines, and also with two nodal lines, so far as they are taking place parallel to the other edge. Sketch and explain the sand figures when («) the corners are in the same and (b) when they are in opposite phases of vibration due to these two components.
Ana.—Let fig. A represent the plate as if vibrating parallel to one edge only, and at the moment when the middle rectangle is above the horizontal (denoted by a) and the lateral rectangles below (denoted by b), the two lines representing the nodal lines. Let fig. B represent the
D.                                E
same plate, as if vibrating parallel to the other edge only, and at the moment when the inner rectangle is above and the outer rectangles below the horizontal lines (denoted by a and b as before). Now let C represent A and B superposed. On examination it is seen that the centre square marked a will have a double amplitude above and the corners b a double amplitude below the horizontal. Also the points