REFLECTION OF SOUND.] 



UNDULATORY FORCES. ACOUSTICS. 



277 



will be readily perceived, through the conducting power 

 of the wood. In a similar manner, a poker, if hung by 

 means of a piece of string held in the teeth, will produce 

 a powerful sound when struck by a hammer or other 

 body. 



A very ingenious application of these principles was 

 made, some years ago, by Professor Wheatstone, of King's 

 College, London. He conceived the idea of conveying 

 musical sounds from any instrument into a distant apart- 

 ment ; so that a "supply" of music might readily be laid 

 on in each room of a house, if rods were attached to any 

 instrument, and fitted with ditfnsers, such as we have 

 illustrated in the last engraving. The plan was practically 

 carried out at the London Polytechnic. In the basement 

 of the building, a pianoforte, cornet, and violin were 

 played ; and from each a deal rod was carried, to a 

 height of 40 feet, into the upper lecture theatre. On 

 each deal rod a harp was placed ; and by these means 

 the sound produced by each instrument was at once 

 diffused. The effect was singular and interesting, for 

 each instrument was distinctly heard by itself, or in 

 concert with the rest ; the music, however, all proceeding 

 from the sound-diffusing harps. In a similar manner the 

 sound of human voices was also conveyed ; the singers 

 being placed with their mouths close to the sounding- 

 board of the piano in the basement, and the harp 

 answering as the ditfuser in the lecture theatre. The 

 Mg engraving shows how the same plan may be 

 carried out in any two apartments in the same house. 

 Fig. 5. 



The pianoforte and the performer are to be placed in a 

 lower apartment, and the audience and diffusing-harp in 

 the upper one. A deal rod rests on the sounding-board 

 of the piano, near C treble ; and the harp rests on its 

 upper extremity. So long as this arrangement is main- 

 tained the performance is distinctly heard ; but if the 

 harp in the upper apartment be removed from the deal 

 rod, the sound instantly ceases. A violin or empty deal 

 box answers just as well as the harp for diffusing tho 

 sound. 



If a musical box or bell be placed in the receiver of 

 an air-pump, their sounds will be audible until the air be 

 exhausted ; but then, owing to the absence of any medium 

 by means of which their vibrations may be conducted, 

 the sounds are entirely lost ; but if the air be allowed 

 again to pass into the receiver, the sound becomes im- 

 mediately perceptible. 



With respect to the greatest distance at which sounds 



can be heard in air, much depends on the state of the 

 atmosphere. Bells of churches on shore, have been heard 

 many miles out at sea ; and the report of cannon has been 

 noticed at astonishing distances. Sound is also well 

 conducted by rocks ; and the noise of the pickaxe of the 

 miner may be heard by another person at a long distance 

 off, owing to the vibrations being conveyed by the solid 

 rock. 



THE REFLECTION OF SOUND. 



THE analogies between heat and sound are very evident, 

 for they can each be conducted, and also reflected, by 

 proper means ; and having spoken of the conduction of 

 sound, we shall now proceed to show by what means it 

 may be reflected : and thereby explain the cause of 

 echoes, and the construction and use of some acoustic 

 instruments. Two concave reflectors, made of tin or 

 zinc, will be required for some of the experiments we 

 shall mention. They may be of a similar sliape and size 

 to those illustrated in a previous page. * 



It may facilitate our progress, if, in future, we employ 

 the term "ray" to the emanations, or waves of sound, 

 proceeding from any source. This will also enable our 

 readers to connect the laws of the reflection of heat and 

 light with those of sound, and also to become acquainted 

 with the analogy existing between them. If a ray of 

 sound impinge on a plane reflector, such as an even wall 

 or a sheet of glass, it will be reflected therefrom in a 

 similar manner to one of heat or light ; and the angle 

 formed by the incident ray will be equal to that of the 

 reflected ray. By pursuing this law, a variety of appli- 

 cations of it may be made in the construction of such 

 instruments as the speaking, hearing- trumpet. >!zc. The an- 

 nexed engraving illus- 

 trates the mode in 

 which the voice of a 

 man may thus bo sent 

 to a great distance by 

 means of the former 

 instrument. The ex- 

 ternal part of the figure 



represents the speak- "-.<* 



ing-trumpet, usually employed on board ships: a a are two 

 rays of sound, which have been reflected from d after 

 proceeding from the mouth, c. Now, had not the re- 

 flecting surface of the trumpet been present at d, then 

 the sound would have radiated off towards e, and only 

 that ray proceeding in the direction c b would have 

 reached the ear of a person at a distance. But by 

 means of this tube, all rays passing from the mouth, c, 

 otherwise than in a line parallel to c b, impinge on the 

 sides of the tube, and are thence reflected therefrom in 

 lines which are parallel to c b ; and so a larger number 

 reach the ear by the use of this arrangement. 



On the same principle, the whispering gallery of St. 

 Paul's Cathedral is constructed. The sound, from any 

 source, impinging on the circular walls, is reflected con- 

 tinuously, and at last reaches the ear at a distance, with 

 a very slight diminution of effect. Echoes are similarly 

 caused by the successive reflection of any soimd from the 

 walls or rocks, placed in such positions with each other, 

 that any ray impinging on them may be readily reflected. 

 Just in proportion to the length of such reflecting sur- 

 faces, and the number of incidences and reflections the 

 sound makes, so the number of echoes occurs. 



The principle of the successive reflection of sound is 



g. e. 



f.. 



d..- 



Fig. 7. 



illustrated by the annexed en- 

 graving, which represents the 

 half of a wluspering gallery. 

 We will suppose a sound to 

 proceed from a to c ; it may 

 then be again reflected from c 

 to d, or to A. Similarly, a ray 

 of sound, proceeding from b, 



may be heard at c b, and reflected again to d or A. If 



the whole circle be formed, then the reflections, whether 



short or long, will be produced over the entire circular 



See ante, p. 27 ; Tig. 6 Section, " Heat." 



