294 LECTURE XXXI. 



ing trumpet has any immediate effect in strengthening the voice, inde- 

 pendently of the reflection of sound. (Plate XXV. Fig. 342.) 



An umbrella, held in a proper position over the head, may serve to collect 

 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 whispering 

 gallery at St. Paul's produces an effect nearly similar, by a continued repe- 

 tition of reflections. Mr. Charles's paradoxical exhibition of the Invisible 

 Girl * has also been said to depend on the reflection of sound ; but the de- 

 ception is really performed by conveying the sound through pipes, artVully 

 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 pro- 

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

 a certain 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 re- 

 servoir, 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 to- 

 wards 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 ad- 

 ditional strength at the expense 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 centre increases ; consequently their energy, which is to 

 be considered as the measure of the strength of sound, must vary as the 

 square of the distance ; so that, at the distance of ten feet from the sounding 

 body, the velocity of the particles of the medium becomes one tenth as 

 great as at the distance of one foot, and their energy, or the strength of the 

 sound, only one hundredth as great. 



LECT. XXXI. ADDITIONAL AUTHORITIES. 



Sound in general. Mersenne, Harmonie Universelle, fol. Paris, 1636. Lahire, 

 Hist, et Mem. de Paris, 1716, p. 262, H. 66. Hales, Doctrina Sonorum, 4to, Lorfd. 

 1778. Dr. T. Young on Sound and Light, Ph. Tr. 1800, p. 106. Huddlestone's 



* See Nich. Jour. 1802, p. 56 ; 1807, p. 69. 



