288 COMPOSITION OF SOUND WAVES. 



cylinder about a foot (30 cm.} long. The cylinder has a spiral 

 groove upon its curved surface and a similar thread upon its axis, 

 which turns in a fixed nut. As the cylinder is turned by a crank, 

 the threads upon the axis give the cylinder a lengthwise motion. 

 The style is placed in position over one of the tinfoil covered 

 grooves of the cylinder. As the cylinder revolves, a projection in 

 front of the style crowds the foil down into the groove. The needle 

 follows in the channel thus made, and, as it vibrates, records a suc- 

 cession of dots in the tinfoil. These dots constitute the record. To 

 the naked eye they look alike, but the microscope reveals differences 

 corresponding to pitch, loudness, and timbre. 



(&.) To reproduce the sound, the style is lifted from the foil, the 

 cylinder turned back to its starting-point, the style placed in the 

 beginning of the groove, and the crank turned. The style passes 

 through the channel and drops into the first yidentation ; the disc 

 follows it. The style rises and drops into each of the succeeding 

 indentations, the disc following its every motion with a vibration. 

 The original vibrations made the dots ; the dots are now making simi- 

 lar vibrations. Sound waves made the original vibrations ; now the 

 reproduced vibrations create similar sound waves. The reproduced 

 sounds are a little muffled but not indistinct, each of the three 

 qualities ( 430) being recognizable. The principle may be applied 

 to any implement or toy that makes a sound as well as to the voice. 

 Perfectly simple ; equally wonderful. 



448. Coincident Waves, In the case of water 

 waves, when crest coincides with crest the water reaches a 

 double height. So with sound waves, when condensation 

 coincides with condensation, this part of the wave will be 

 more condensed ; when rarefaction coincides with rarefac- 

 tion, this part of the wave will be more rarefied. This 

 increased difference of density in the two parts of the wave 

 means increased loudness of the sound, because there is an 

 increased amplitude of vibration for the particles consti- 

 tuting the wave. 



449. Reinforcement of Sound. This increased 

 intensity may result from the blending of two or more 

 series of similar waves in like phases, or from the union of 



