BY COLOURED MEDIA. 481 



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 

 continual 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 reflec- 

 tions from its surface to be), it becomes frittered away 

 to an insensible amplitude, and resolved into innumer- 

 able others ; crossing, recrossing, and mutually compen- 

 sating each other, while each of the secondary waves so 

 produced is in its turn undergoing the same process of 

 disruption and degradation. 



(6.) In this account of the apparent destruction of 

 motion, I have purposely supposed the body set in vibra- 

 tion 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 tremors communicated 

 to the air. But in the case of an inelastic or imperfectly 

 elastic body, the internal process above described goes 

 on with such excessive rapidity, as to allow of very few, 

 and those rapidly degrading, impulses to be communi- 

 cated from its surface to the air. 



(7.) In my Article on Sound,* I have explained, on this 

 principle of internal reflection and continual subdivision, 



* Published like that on Light, above cited, in the Encyclopedia 

 Metropolitan^ 1829-30. 



2H 



