52 PRACTICAL LESSONS IN SCIENCE. 



and furrows called waves, which extend onward and onward, 

 finally breaking against the shore. The ridges and furrows 

 move outward from the point of disturbance, but the water only 

 moves upward and downward; that is, it vibrates. Another 

 stone dropped into the water a little distance from the first sets 

 up another series of waves. These waves interfere with the waves 

 caused by the first stone, breaking up the symmetry of each, cut- 

 ting the surface of the water into curious shaped elevations and 

 depressions having no semblance to uniformity. Sometimes the 

 elevation of one group of waves is piled upon the elevation of 

 the other group, making waves nearly twice as high as a single 

 wave of either group ; and sometimes the furrows of one group 

 correspond with the furrows of the other, making a much deeper 

 furrow than a single wave of either group; and again, a ridge of 

 one set will correspond with the furrow of another set, and a 

 level surface will result. 



Waves may be caused in the air as in the water, but they differ 

 in form. The waves of water are circular, while the air waves are 

 like spherical shells, spreading out in every direction. The air 

 particles simply move backward and forward, as the water par- 

 ticles move upward and downward. Sometimes the vibrations of 

 the air are intense enough, so that, acting on the organs of hear- 

 ing, they give rise to sensation of sound. Single intense air 

 waves cause a sound called a report ; a series of such sounds in 

 irregular succession makes a noise, and when the waves succeed 

 each other so rapidly and regularly that there seems to be no in- 

 terval between them, the result is a musical sound or tone. In 

 general, less than from thirty -two to forty vibrations in a second 

 give rise to reports or noises, but vibrations, varying in number 

 from 40 to 40,000 per second, can ordinarily be appreciated as 

 musical tones. 



The tones of the voice and of many musical instruments de- 

 pend upon the vibrations of chords that are subject to more or 

 less tension. Lay a brick or a piece of wood about the size of a 

 brick upon each end of a table. Tie a string to one leg of the 

 table, pass it over the bricks, and tie it to another brick as a 



