respecting Sound and Light. 135 



the inflexibility of a wire would dispose it, according to the doc- 

 trine of elastic rods, to assume the form of the harmonic curve. 

 The theorem of Euler and De la Grange, in the case where 

 the chord is supposed to be at first at rest, is in effect this : con- 

 tinue the figure each way, alternately on different sides of the 

 axis, and in contrary positions; then, from any point of the curve, 

 take an absciss each way, in the same proportion to the length 

 of the chord as any given portion of time bears to the time of 

 one semivibration, and the half sum of the ordinates will be the 

 distance of that point of the chord from the axis, at the expira- 

 tion of the time given. If the initial figure of the chord be com- 

 posed of two right lines, as generally happens in musical 

 instruments and experiments, its successive forms will be such 

 as are represented in Plate VI. Figs. 47, 48 : and this result is 

 fully confirmed by experiment. Take one of the lowest strings 

 of a square piano forte, round which a fine silvered wire is wound 

 in a spiral form ; contract the light of a window, so that, when 

 the eye is placed in a proper position, the image of the light may 

 appear small, bright, and well defined, on each of the convolu- 

 tions of the wire. Let the chord be now made to vibrate, and 

 the luminous point will delineate its path, like a burning coal 

 whirled round, and will present to the eye a line of light, which, 

 by the assistance of a microscope, may be very accurately ob- 

 served. According to the different ways by which the wire is 

 put in motion, the form of this path is no less diversified and 

 amusing, than the multifarious forms of the quiescent lines of 

 vibrating plates, discovered by Professor Chladni; and is indeed 

 in one respect even more interesting, as it appears to be more 

 within the reach of mathematical calculation to determine it ; 

 although hitherto, excepting some slight observations of Busse 



