t/ferrt^ 



S76 LECTURE XXXI. 



the force of a distant sound by reflection, in the manner of a hearing 

 trumpet; but its substance is too slight to reflect any sound very perfectly, 

 unless the sound fall on it in a very oblique direction. The whisperino- o-al- 

 lery at St. Paul's produces an effect nearly similar, by a continued repetition 

 of reflections. Mr. Charles's paradoxical exhibition of the Invisible Girl has 

 also been said to depend on the reflection of sound; but the deception is 

 really perfonned by conveying the sound through pipes, artfully concealed, 

 and opening opposite to the mouth of the trumpet, from which it seems to 

 proceed. 



When a portion of a pulse of sound is separated by any means from the 

 rest of the spherical or hemispherical surface to which it belongs, and proceeds 

 through a wide space, without being supported on either side, there is a cer- 

 tain degree of divergence, by means of which it sometimes becomes audible 

 in every part of the medium transmitting it: but the sound thus diverging 

 is comparatively very faint; and more so indeed than the effect of a wave 

 of water, admitted under similar circumstances, into a wide reservoir, which 

 we have already examined. Hence, in order that a speaking trumpet may 

 produce its full effect, it must be directed in a right line towards the hearer: 

 and the sound collected into -the focus of a concave mirror is far more 

 powerful than at a little distance from it, which could not happen if, as 

 .some have erroneously supposed, sound in all cases tended to spread equally 

 in all directions. The sounds that enter a room, in which there is an open 

 window, are generally heard by a mixture of this faint divergence with the 

 reflection from various parts of the window and of the room, and with the 

 effect of the impulse transmitted through the walls. This diverging portion, 

 however faint, probably assists in preserving the rectilinear motion of the 

 principal sound, and gradually gains a little additional strength at the ex- 

 pense of this portion. 



The decay of sound is the natural consequence of its distribution through- 

 out a larger and larger quantity of matter, as it proceeds to diverge every 

 way from its centre. The actual velocity of the particles of the medium 

 transmitting it appears to diminish simply in the same proportion as the 

 distance from the <?entre increases; consequently their energy, which is to 



