142 SYMPATHETIC VIBRATION. SECT. XVII 



elasticity, or tendency to resume their natural form, a 

 series of undulations takes place, owing to the alternate 

 condensations and rarefactions _of the particles of solid 

 matter. These have also their harmonic tones, and 

 consequently nodes. Indeed generally, when a rigid 

 system of any form whatever vibrates either transverse- 

 ly or longitudinally, it divides itself into a certain number 

 of parts, which perform their vibrations without disturb- 

 ing one another. These parts are at eveiy 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 trans- 

 mitting its undulations when in contact with a body sus- 

 ceptible 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 harmonic divisions. If two chords 

 equally stretched, of which one is twice or three times 

 longer than the other, be placed side by side, and if the 

 shorter be sounded, its vibrations will be communicated 

 by the air to the other, which will be thrown into such 

 a state of vibration that it will be spontaneously divided 

 into segments equal in length to the shorter string. 

 When a tuning-fork receives a blow and is made to rest 

 upon a piano-forte during its vibration, every string 

 which, either by its natural length or by its spontaneous 

 subdivisions, is capable of executing corresponding vibra- 

 tions, responds in a sympathetic note. Some one or 

 other of the notes of an organ are generally in unison 

 with one of the panes or with the whole sash of a win- 

 dow, which consequently resounds when these notes 

 are sounded. A peal of thunder has frequently the 

 same effect. The sound of very large organ-pipes is 

 generally inaudible till the air be set in motion by the 

 undulations of some of the superior accords, and then 

 its sound becomes extremely energetic. Recurring vi- 

 brations occasionally influence each other's periods. For 

 example, two adjacent organ-pipes nearly in unison, may 

 force themselves into concord ; and two clocks whose 

 rates differed considerably when separate, have been 



