COMPOSITION OF SOUND WAVES. 285 



(c.) When the two forks are in unison, their periods are the same. 

 The second and subsequent pulses sent out by the first fork strike 

 the second fork, already vibrating from the effect of the first pulse, 

 in the same phase of vibration, and thus each adds its effect to that 

 of all its predecessors. If the forks be not in unison, their periods 

 will be different and but few of the successive pulses can strike 

 the second fork in the same phase of vibration ; the greater number 

 will strike it at the wrong instant. 



444. Soundiiig-Boards. In the case of the 

 sonometer, piano, violin, guitar, etc., the sound is due 

 more to the vibrations of the resonant bodies that carry 

 the strings than to the vibrations of the strings them- 

 selves. The strings are too thin to impart enough motion 

 to the air to be sensible at any considerable distance ; but 

 as they vibrate, their tremors are carried by the bridges to 

 the material of the sounding apparatus with which they 

 are connected. 



(a.) This sounding apparatus usually consists of thin pieces of 

 wood which are capable of vibrating in any period within certain 

 limits. The vibrations of these large surfaces and of the enclosed 

 air produce the sonorous vibrations. The excellence of a Cremona 

 violin does not lie in the strings, which may have to be replaced 

 daily. The strings are valuable to determine the rate of vito'ation 

 that shall be produced ( 455). The excellence of the instrument 

 depends upon the sonorous character of the wood, which seems to 

 improve with age and use. 



(6.) Similar remarks apply to the tuning-fork, When a tuning- 

 fork held in the hand is struck, but a feeble sound is heard. When 

 the handle is placed upon the table or almost any solid having a 

 considerable surface, the intensity of the sound is remarkably in- 

 creased. Hence, for class or lecture experiments, tuning forks 

 should be mounted as shown in Fig. 223. 



Note. Before beginning the study of the telephone, the pupil 

 should carefully review 408, 409. 



445, The Telephone. This instrument is repre- 

 sented in section by Fig. 224. A is a permanent bar 



