30 



PNEUMATICS. 



tremity of a cylindrical tube, upwards of 

 3000 feet in length, a ring of metal was 

 placed, of the same diameter as the 

 aperture of the tube ; and in the centre 

 of this ring, in the mouth of the tube, 

 was suspended a clock-bell and hammer. 

 The hammer was made to strike the 

 ring and the bell at the same instant, so 

 that the sound of the ring would be 

 transmitted to the remote end of the 

 tube, through the conducting power of 

 the matter of the tube itself; while the 

 sound of the bell would be transmitted 

 through the medium of the air included 

 within the tube. The ear being then 

 placed at the remote end of the tube, 

 the sound of the ring, transmitted by 

 the metal of the tube, was first distinctly 

 heard ; and after a short interval had 

 elapsed, the sound of the bell, transmit- 

 ted by the air in the tube, was heard. 

 The result of several experiments was, 

 that the metal of the tube conducted the 

 sound with about ten and a half times 

 the velocity with which it was conducted 

 by the air ; that is, at the rate of about 

 11,865 feet per second. 



(56.) Sound is reflected from hard and 

 smooth surfaces, according to laws simi- 

 lar to those which govern the reflection 

 of light ; and, similar to light, it is pro- 

 pagated in right lines. 



Let A (fig. 31.) be the position of a 

 sounding body, and let B C be a smooth 

 fig. 31. 



and hard surface at the distance AD, 

 the line A D being perpendicular to B C. 

 The sound is propagated in right lines 

 diverging from A, and the rays of sound 

 strike the surface B C at the points E, 

 F, G, H, I, &c. They are then reflected 

 from the surface B C, at angles equal to 

 those at which they strike it; that is, 

 the angle DEA is equal to B E K, 

 DFA is equal to B F L, DGAto 

 B G M, &c. Now, if this be the case, 

 by a well-known geometrical theorem, 

 the lines K E, L F, M G, N H, &c., if 

 continued back in the directions K E, 

 L F, &c., will all meet in a point A', as 

 fear behind the surface B C, as A is be- 

 fore it ; so that A D = A'D. The rays 



of sound, EK, F L, GM, &c., will 

 therefore proceed, as if they emanated 

 from a sounding body placed at A'. 

 These rays of sound will therefore affect 

 an ear placed any where within their 

 range, as at X, exactly as if the sound- 

 ing body were placed at A' ; and if a 

 sufficient number of these reflected rays 

 meet the ear at X, the reflected sound 

 will be heard. But the sound of A will 

 be first heard in the direction of the line 

 A X, so that a repetition or echo will be 

 the effect. The line X A being less than 

 X A, the direct sound in the line X A 

 will be first heard ; and after an interval, 

 equal to the time which sound takes to 

 move through a space equal to the dif- 

 ference between the distances X A and 

 X A', the echo will be heard. 



When there is but one reflecting sur- 

 face, it seldom happens that a sufficient 

 number of rays of sound meet the ear 

 to produce sensation, in which case no 

 echo will be perceived. But if the ear 

 be placed at the sounding body, and 

 if smooth and hard surfaces be 'placed 

 in various directions round this centre, 

 they will severally reflect back the 

 sound. In order, however, that sensa- 

 tion should be produced, it will be 

 necessary that a number of these reflec- 

 tions should reach the ear at the same 

 instant. This will necessarily be the 

 case if a number of the reflecting sur- 

 faces are at equal distances from the 

 ear and the sounding body. If, then, 

 the place of the ear and the sounding 

 body be the centre of a circle, and if 

 in the circumference of this circle and 

 at right angles to lines drawn from the 

 centre, a number of plane reflecting sur- 

 faces be placed, the rays of sound pro- 

 ceeding from the centre will be reflected 

 back to the centre, so as to produce a 

 distinct perception of the sound or an 

 echo. The number of seconds between 

 the production of the sound and its 

 echo may be found by dividing twice 

 the number of feet in the radius of the 

 circle by 1130. 



Let C (^.32.)be the place of the audi- 

 tor, and A, B, D, E, F be plane reflect- 

 ing surfaces placed in the circumference 

 of the same circle. The sound produced 

 at C moves along C A, and being re- 

 flected at, A, returns along A C, and 

 arrives at C after as many seconds as 

 1130 feet are contained in twice AC. 

 Since the lines AC, B C, D C, E C, 

 F C are equal, the sound is reflected 

 from the surfaces B, D, E, Fin exactly 

 the same time as from A. Now, al- 



