ACOUSTICS. 



m transmuting it : hi. t the sound 

 ihus diverging is co:np;r 

 Hence, in onler that a speaking-trumpet 

 may produce its full effect, it must be di- 

 rected in u r 



and the sound col.ci tc.l into the focus of 

 a concave mirror 



lhu;i :>t u little distance from it, which 

 could not happen, it' sound, in al: 

 tended to spread equally in all directions . 

 It is said tlr.il the report of a cannon ap- 

 pears many times louder to a per-on '<>- 

 u horn it is tired, than to on.- plared 

 in a contrary direction. It must, says Dr. 

 Young, have occurred to (very one's oh. 

 -ervation, that a sound, such as that of a 

 mill, or a tall of water, has appeared much 

 louder after turning 1 a corner, when the 

 house or other obstacle no longer inter- 

 vened. Indeed, the whole theory of the 

 sp< -ik ing-trumpet would fall to the ground, 

 if it wen- demons! rablc that sound spreads 

 equally in all directions. Jn windy wea- 

 ther, it may he often observed, that the 

 .sound of a distant hell varies almost in- 

 stantaneously in its strength, so as to ap- 

 pear twice us remote at one time as an- 

 other. Now, if sound diverged equally in 

 all directions, the variation produced by 

 the wind would not exceed one-tenth of 

 the apparent distance ; but on the suppo- 

 sition of a motion nearly rectilinear, it may 

 asii\ happen that a slight change in the 

 direction of the wind shall convey u sound, 

 either directly or after reflection, in very 

 different degrees, to the same spot. 



The decay of sound is the natural con- 

 sequence of its distribution throng-horn a 

 larger ud larger quantity of matter, as it 

 dsto diverge every wa\ from its 

 centre. The actual velocity of the parti- 

 . i< -s of the medium transmitting 1 it, appears 

 to diminish,. simply, in t h'- sanir proportion 

 as the distance from the centre inc; 

 consequently, their energy, which is to be 

 considered as the measure ofthe strength 

 i>f sound, must \ar\ as the square of the 

 : so that, at the distance- of ten 

 tcet from the sounding lx>d\, the \elocity 

 ofthe particles of the medium becomes 

 one-tenth as great as at the distance of one 

 loot. Muhlieir energy, or the strength of 

 the sound, only one-hundredth as great. 



An echo is a reflection of sound strik- 

 ing against some object, as :>.n imagi i-> 

 rilected in a ghiss : but it has been dis- 

 puted, what are the proper qualitic-, in a 

 'iody for thus reflecting sounds. It is in 

 g'-neral known, that caverns, grottoes, 

 mountains, and ruined buildings, return 

 this reflection of sound. \Ve have heard 

 of a very extraordinary eho, at a ruined 



VOL. I. 



fortress near Louvuin, in. Flanders. If a 

 pei-son sung, he only heard his own \nicj-. 

 without any repetition ; on the contrary, 



.ho slood at some distance heard 

 the echo, but not the void.- ; but then they 

 heard it with surprising variations, 

 times louder, som. -times softer, now more 

 near, then more distant. There is an ac- 

 count, in the .fthe French aca- 

 demy, of a sim.lar echo near Rouen. It 

 observed, that every 

 point against which the pulses of sound 

 strike becomes the centre of a new series 

 of pulses, and sound describes equal dis- 



'ii equal times; therefor--, \\hc!i 

 any sound is propagated from a centre, 

 and its pulses strike against a variety of 

 obstacles, if the sum of the right lines 

 di-.iwn from that point to each oi' the ob- 

 stacles, and from each obstacle to a second 

 point, be equal, then will the latter be a 

 point in which an echo will be heard. 

 Thus, let A, fig. 4, bethe point from which 

 the sound is propagated in all directions, 

 anil let the pulses strike against the ob- 

 stacles C, D, E, F, G, H, 1, &c. each of 

 these points becomes a ne\v centre of pul- 

 ses by the first principles, and therefore 

 from each of them one series of pulses will 

 pass through the point B. Now, if the 

 several sums of the right lines A C 4- C B, 

 A n + D~BT A~E -f E B, A G + G B, 

 A H f H B, A 1 -f- 1 B, &c. be all equal 

 to euch other, it is obvious that the pulses 

 propagated from A to these point-., and 

 again from these points to B, will all ar- 

 n\e at H at the same instant, accordingto 

 the second principle ; and, therefore, if 

 the hearer hf in that point, his ear will at 

 the same instant be struck by all these 

 pulses. Now it ap pears, from experiment, 

 that the ear of an exercised musician can 

 alone distinguish such sounds as follow 

 one another at the ru'.e of 9 or 10 in a se- 

 cond, or any slower rate ; and therefore, 

 for a distinct perception of the direct and 

 reflected sound, there should intervene 

 the interval of 1 of a second; but in this 



time sound describes g- or 127 feet 



nearly. And therefore, unless i|n- sum of 

 the lines drawn from each of the obi 

 to tlie points A and I! exceeds the interval 

 AH b) KT feet, no echo will be heard at, 

 II. Sun e tin several sums of the I'nes 

 drawn from the obstacles to the points \ 

 and B are of the same magnitude, it ap- 

 pears that the curve passing- thro 1 

 the points. ( \ l ), K, F, G, II, l,,Vc. will be 

 an ellipse. Hence all the points of the 

 obstacles which produce an echo 



