164 SYMPATHETIC VIBRATION. [SECT. xvn. 



only, that, when the tube or rod is continually turned on its 

 axis in the same direction, the rings slide along during the 

 vibrations, till they come to a quiescent point, where they 

 rest. By tracing these nodal lines he discovered that they 

 twist in a spiral or corkscrew round rods and cylinders, 

 making one or more turns according to the length ; but at 

 certain points, varying in number according to the mode of 

 vibration of the rod, the screw stops, and recommences on 

 the other side, though it is turned in a contrary direc- 

 tion ; that is, on one side it is a right-handed screw, on 

 the other a left (N. 183). The nodal lines in the interior 

 surface of the tube are perfectly similar to those in the ex- 

 terior, but they occupy intermediate positions. If a small 

 ivory ball be put within the tube, it will follow these nodal 

 lines when the tube is made to revolve on its axis. 



All solids which ring when struck, such as bells, drinking 

 glasses, gongs, 1 <fec., have their shape momentarily and 

 forcibly changed by the blow, and from their elasticity, or 

 tendency to resume their natural form, a series of undula- 

 tions take place, owing to the alternate condensations and 

 rarefactions of the particles of solid matter. These have also 

 their harmonic tones, and consequently nodes. Indeed, ge- 

 nerally, when a rigid system of any form whatever vibrates 

 either transversely or longitudinally, it divides itself into 

 a certain number of parts which perform their vibrations 

 without disturbing one another. These parts are at every 

 instant in alternate states of undulation ; and, as the points 

 or lines where they join partake of both, they remain at rest, 

 because the opposing motions destroy one another. 



The air, notwithstanding its rarity, is capable of transmit- 

 ting its undulations when in contact with a body susceptible 

 of admitting and exciting them. It is thus that sympathetic 

 undulations are excited by a body vibrating near insulated 

 tended strings, capable of following its undulations, either by 

 vibrating entire, or by separating themselves into their har- 

 monic divisions. If two chords equally stretched, of which 



