290 



THE POPULAR EDUCATOR. 



rays of light and heat; like them, they are capable of being 

 reflected, and even refracted, by the employment of suitable 

 lenses. A good and simple experiment, to illustrate the former 

 of these facts, can easily be tried with an ordinary concave 

 mirror. Having placed a bright light at one side of the room, 

 place the mirror opposite it, and, by holding some object in 

 front of it, ascertain the point where the image of the light is 

 formed that is, the focus of the mirror. Now remove the light, 

 and in its place suspend a watch. Owing to the distance, its 

 round will probably be inaudible to a person standing near the 

 mirror; but let him place his ear at the focus, and he will at 

 once distinctly hear it. As it is somewhat difficult to find the 

 exact focus for the ear, the experiment will succeed better if a 

 glass funnel be held at the focus, and the ear applied to the tube 

 of this. The sonorous waves will thus be more fully collected 

 and conveyed to the ear. The funnel in this case fulfils a similar 

 purpose to a hearing trumpet, the waves being reflected from 

 side to side till they travel down the tube. 



If two concave mirrors be employed instead of the one, the 

 watch being placed in the focus of the one, and the funnel at 

 that of the other, the sound will be heard at a much greater 

 distance. Curved roofs and ceilings sometimes act in this way, 

 and reflect the sound, and hence the ceilings of large buildings 

 have usually a vaulted form. The arch of a bridge acts simi- 

 larly, and two persons, properly placed under it, may often hold 

 conversation with one another in tones so low, that they 

 are totally inaudible to a third individual standing between 

 them. 



In a similar way, two people situated in the foci of large con- 

 cave mirrors may hold conversation with one another at a great 

 distance. Two such mirrors, about six feet in diameter and 

 about one hundred feet apart, were some time ago at opposite 

 ends of the large hall in the Polytechnic Institution at London. 

 It was found, however, that even when the hall was filled with 

 people, and there was much noise, whispers uttered by those 

 in the focus of one could be heard by a listener in the focus 

 of the other. 



The well-known whispering gallery of St. Paul's is another 

 good illustration of the reflection of sound. The wall surround- 

 ing the gallery is circular and smooth, and hence the vibrations 

 produced by the voice, instead of being dissipated in space, are 

 reflected from spot to spot, till they reach the ear of the listener 

 at the opposite side. 



Fig. 14 will explain the manner in which the waves of sound 

 are thus reflected by any smooth surface against which they 

 strike. If the surface be rough or uneven, they will, of course, 

 be irregularly broken up and scattered, just as the rays of 

 light which fall upon an uneven surface are irregularly 

 diffused. 



If A be the position of a sounding body, the waves of sound 

 produced by it will be represented by concentric circles, so long 

 as nothing intervenes to interfere with their motion. They 

 soon, however, reach the obstacle P Q, where their course is 

 arrested. The sound first meets this obstacle on the line A a, 

 and is thrown back, so that the wave, M C D N, has its middle 

 portion deflected into the arc c K D. The wave which reaches 

 any point, c, will have travelled in the direction A c ; but since 

 the angle of incidence is equal to that of reflection, it will con- 

 tinue its course in the direction c B, and, to an observer at B, 

 will appear to have proceeded from the point a, situated as far 

 behind P Q as A is in front of it,. By considering in a similar 

 way the waves that meet each portion of the obstacle, we shall 

 find that the curve, C K D, in which the wave continues to move, 

 after being reflected, is in reality an arc of a circle, whose centre 

 is art the point a. The laws of the reflection of sound are thus 

 exactly the same as those for the reflection of light, and need 

 not, therefore, be further explained. 



An echo in a repetition of a sound caused by the waves being 

 reflected to the ear from some obstacle, as, for instance, a cliff 

 or a lofty wall. If the reflecting surface be very near, a distinct 

 echo will not be produced, as the sound will return so quickly as 

 to mingle with the original one, and merely render it somewhat 

 indistinct. This effect is well seen in speaking in a large empty 

 room, where the reflection from the walls will frequently render 

 the words almost unintelligible. When the room is filled, the 

 sound-waves are so confused and absorbed, that this effect is 

 much lessened. Curtains are frequently suspended in large 

 rooms for the purpose of further damping the echo. 



A sharp sound, like a blow, may produce an echo when tiie 

 reflecting surface is about fifty or sixty feet distant. In order 

 to produce a syllable, it should bo nearly as far again, and in 

 those cases where two or three syllables are repeated, its dis- 

 tance will be found much greater. 



If there are two reflecting surfaces almost parallel as, for 

 example, two high walls sufficiently distant the sound may be 

 repeated many times. The waves are alternately reflected from 

 one to the other, gradually becoming fainter and fainter, till the 

 sound ceases to be audible. In some places, in mountainous 

 regions, the report of a pistol or the sound of a horn is in this 

 way repeated many times in a most marvellous manner. An 

 echo in the chateau of Simonetta, in Italy, is said to repeat a 

 sound twenty or thirty times. 



When a long tube has a smooth interior, any sound uttered at 

 one end is conveyed along it with little diminution the waves 

 are reflected from side to side, not being dissipated to any appre- 

 ciable extent. M. Biot conducted many experiments of this 

 nature, and found that a conversation might very easily be kept 

 up through water-pipes upwards of one thousand feet in length. 

 Advantage is taken of this fact in the construction of speaking 

 tubes. In many business establishments metal tubes are laid 

 down from one room to another ; a whistle is usually placed in 

 each end, and the person who wants to call the attention of the 

 other first blows through his tube so as to sound the whistle, 

 and then can converse with the other, applying his ear to the 

 tube so as to hear the reply. 



A somewhat similar apparatus is fixed in many churches and 

 chapels to enable deaf people to hear. The tube, which is 

 usually made of gutta-percha, ends in a large funnel-shaped 

 mouth placed against the book-board of the pulpit, and the 

 sound is thus conveyed through the tubes to any part of the 

 building. A flexible piece is usually fitted to the other end to 

 enable the deaf person to put it close to his ear. 



The ordinary hearing trumpet acts in a similar way, the open 

 end of it arresting a large number of the waves, and by its form 

 concentrating them to a focus, and thus greatly increasing the 

 intensity of the sound. 



Waves of sound may be refracted by means of suitable lenses. 

 A balloon of collodion or thin india-rubber, filled with carbonic 

 acid gas, answers well for this purpose. It may be circular in 

 form, or else composed of two segments fastened together at 

 the edges, so as to resemble an ordinary lens. If a watch be 

 held at a little distance from one side, a point may easily be 

 found on the other side ai< which the sound will be collected 

 to a focus, and where it will be distinctly heard. When the ear 

 is removed from this spot, the ticks become much more faint. 



LESSONS IN ITALIAN. XLIL 



VERBS WITH A AFTER INFINITIVE. 



Rule 69. The following verbs have a after them before a 

 following infinitive : 



