72 Royal Institution : — 



note of the same pitch. The same holds good when a vibrating fork 

 is substituted for the vibrating hand. While, therefore, when the 

 vibrations are longitudinal a complete vibration of the fork synchro- 

 nizes with a semi-vibration of the string, when the vibrations are 

 transversal, a complete vibration of the fork corresponds to a com- 

 plete vibration of the string. 



25. Hence, if the string vibrate as a whole when the vibrations 

 are longitudinal, it will divide itself into two ventral segments when 

 the vibrations are transversal ; or, more generally expressed, pre- 

 serving the tension constant, whatever be the number of ventral 

 segments produced by the fork when its vibrations are longitudinal, 

 twice that number are produced when the vibrations are transverse. 

 Here, for example, is a string 8 feet long, vibrating longitudinally ; 

 the fork divides the string into four equal vibrating parts. I place the 

 fork so that it shall vibrate at right angles to the string. The num- 

 ber of ventral segments is now eight, or double the former number. 

 This result was amply illustrated by the experiments of M. Melde. 



26. Attaching two strings of the same length to the two prongs 

 of the same fork, stretching one string in the direction of the prongs, 

 and the other at right angles to that direction, and subjecting the 

 strings to the same tension, when the fork is caused to vibrate, the 

 one string divides itself into double the number of vibrating seg- 

 ments exhibited by the other. 



27. When white silk strings vibrate thus, the beauty of the ven- 

 tral segments is extreme. The nodes appear white and fixed, while 

 the vibrating parts form delicate gauzy spindles in the air ; every 

 protuberance of the twisted string writes its motion in a more or less 

 luminous line on the surface of the aerial gauze. 



28. Certain twisted strings do not adhere to the same plane. 

 They vibrate simultaneously in two rectangular directions, and every 

 one of their points describes a closed curve. When the two vibra- 

 tions are of the same amplitude, and have the proper difference of 

 phase, the ventral segments describe surfaces of revolution. 



29. The path described by the various points of the string may 

 be studied, after the manner of Dr. Young, by throwing light upon 

 them and watching the lines described by the illuminated points. 

 By twisting a flat and burnished silver wire we form a spiral sur- 

 face, from which at regular intervals the light flashes when the whole 

 wire is illuminated. Attaching the wire to a tuning-fork and caus- 

 ing it to vibrate, its luminous spots describe luminous lines. If the 

 wire vibrate as a whole without any sensible admixture of its har- 

 monic divisions, these spots describe straight lines, which are drawn 

 brilliantly across the ventral gauze ; but when, by slackening or 

 tightening the wire, other vibrations are caused to mingle with the 

 fundamental one, the combination of vibrations expresses itself in 

 luminous scrolls of extraordinary beauty. 



30. But to see this beauty you must be close to it. The vibra- 

 ting segments of our silk strings were, I fear, also too faint to be 

 seen at a distance. For the silk string I substitute a fine platinum 

 wire, which now stretches from this tuning-fork over a bridge of 

 copper, and then round a peg. I send an electric current through 



