404 Sir John F. W. Herschel on the Absorption 



perfect elasticity resolves our conception of the vibrating mass 

 into that of a multitude of inharmonious systems communi- 

 cating with each other. At every transfer of an undulation 

 from one such system into that adjacent, a partial echo is 

 produced. The unity of the propagated wave is thus broken 

 up, and a portion of it becomes scattered through the interior 

 of the body in dispersed undulations from each such system, 

 as from a centre of divergence. In consequence of the con- 

 tinual repetition of this process, after a greater or less number 

 of passages to and fro of the original wave across the body, 

 (however perfect we may suppose the reflections from its sur- 

 face to be,) it becomes frittered away to an insensible ampli- 

 tude, and resolved into innumerable others ; crossing, re- 

 crossing, and mutually destroying each other, while each of 

 the secondary waves so produced is in its turn undergoing the 

 same process of disruption and degradation. 



In this account of the destruction of motion, I have pur- 

 posely supposed the body set in vibration to be insulated from 

 communication with any other. In the case of a perfectly or 

 highly elastic body struck in air, it will vibrate so long that a 

 great part of its motion is actually carried off in sonorous tre- 

 mors communicated to the air. But in the case of an inelastic 

 or imperfectly elastic body, the internal process above de- 

 scribed goes on with such excessive rapidity, as to allow of 

 very few, and those rapidly degrading, impulses to be com- 

 municated from its surface to the air. 



In my Essay on Sound, I have explained, on this prin- 

 ciple of internal reflection and continual subdivision, in a me- 

 dium consisting of loosely aggregated earth intermixed with 

 much air, the hollow sounds which are often attributed to the 

 reverberation of subterranean cavities, and in particular the 

 celebrated instance of this kind of sound heard at the Solfaterra 

 near Pozzuoli. The dull and ill-defined sound thus produced 

 from a succession of partial echoes is there assimilated to the 

 nebulous light which illuminates a milky medium when a 

 strong beam is intromitted. If we suppose, now, such a mass 

 of materials insulated from communication with the external 

 air by some sound-tight envelope, these partial echoes, when 

 they reach the surface in any direction, will be all sent back 

 again as so many fresh impulses, till at length it will become 

 impossible to assign a point within the mass which will not be 

 agitated at one and the same moment by undulations traversing 

 it in every possible phase and direction. Now the state of a 

 molecule, under the influence of an infinite number of contra- 

 dictory impulses thus superposed, is identical with a state of 

 rest. 



