376 Dynamic Theory. 



ing to its elasticity. If the body is comparatively in-elastic, the vibra- 

 tion which it communicates to the air is momentary and it is called a 

 noise. But if the body is elastic, it will give out a number of vibrations 

 in regular time, atad if the rapidity of them is so great that the impression 

 made upon our auditory apparatus by one has not time to subside before 

 it is assailed by the next, and the next, the effect is to produce the continu- 

 ous sound which we call musical. Thus, noise is a mathematical point 

 while music is a line. When a clapper strikes a bell a single blow it 

 knocks it out of its circular form and compels it to take an elliptical 

 form, with the long diameter in a direction at right angles to the direc- 

 tion of the blow. The stroke of the hammer creates one sound followed 

 by the sound communicated to the air by the sudden change in the shape 

 of the bell. The strained condition of the bell causes it to fly back to- 

 ward its circular shape, but it flies further and takes an elliptical shape 

 again with its long diameter at right angles to its first position, and so 

 it oscillates from one position to the other a great number of times, each 

 time communicating a vibration to the air. These vibrations follow each 

 other so rapidly as to seem to our ears a continuous sound. Each 

 strained position of the bell is a position of potential energy, and the 

 force with which it descends from one such position is partly consumed 

 in putting it in another. The part not so consumed is expended in the 

 strokes upon the air which constitute sound and in work against the 

 cohesion of the molecules of the bell which disappears as heat. These 

 two gradually use up the energy of the stroke of the clapper so that the 

 oscillations become less and less and finally cease. The molecular con- 

 stitution and form of every elastic body determine the length of the 

 vibration ( or the pitch of the sound ) it will communicate to the air when 

 it is struck. Thus if an elastic string be stretched with a certain tension 

 it will always give a certain sound when vibrated. If the force applied 

 is strong the tone will be loud, but the pitch will remain the same as 

 when the vibration is soft. The rapidity of the vibrations per second 

 then is the same in both cases. The same law governs molecular vibra- 

 tions that governs the swinging of the pendulum. A pendulum of a 

 certain length always makes the same number of oscillations per second 

 regardless ( within limits \ of the length of the arc through which it 

 swings. When air in a tube or crack or any confined place is caused to 

 vibrate whether loudly or softly the pitch is always the same for the 

 same body of air thus confined. 



This pitch is called its fundamental tone. Every simple body that 

 vibrates at all and very few do not has its fundamental tone, which 

 it emits on being struck. Every piano string, organ pipe, jewsharp, 

 horn, kettle, pan, key-hole, bar of iron, strip of glass, bell, &c. , has its 

 fundamental note, and it will give only this when struck. A great 



