722 SPECIAL SENSES 



ments and of different registers of the human voice, as in choral and 

 orchestral compositions, shades of effect, almost innumerable, may be 

 produced. The modification of sounds in this way constitutes harmony ; 

 and an educated ear not only experiences pleasure from these musical 

 combinations, but can distinguish their different component parts. 



A chord may convey to the ear the sensation of completeness in 

 itself or it may lead to a succession of notes before this sense of com- 

 pleteness is attained. Different chords in the same key may be made 

 to follow each other, or by transition-notes, may pass to the chords of 

 other keys. Each key has its fundamental note ; and the transition from 

 one key to another, in order to be agreeable to the ear, must be made 

 in certain ways. These regular transitions constitute modulation. The 

 ear becomes fatigued by long successions of notes or chords always in 

 one key, and modulation is essential to the enjoyment of elaborate 

 musical compositions ; otherwise the notes would not only become 

 monotonous, but their correct appreciation would be impaired, as the 

 appreciation of colors becomes less distinct after looking for a long 

 time at an object presenting a single vivid tint. 



Laws of Sonorous Vibrations. Sound is produced by vibrations in 

 a ponderable medium ; and the sounds ordinarily heard are transmitted 

 to the ear by means of vibrations of the atmosphere. A simple and 

 very common illustration of this fact is afforded by the experiment of 

 striking a bell carefully arranged in vacuo. Although the stroke and 

 the vibration can readily be seen, there is no sound ; and if air is grad- 

 ually introduced, the sound will become appreciable and progressively 

 more intense as the surrounding medium is increased in density. The 

 oscillations of sound are to and fro in the direction of the line of con- 

 duction and are said to be longitudinal. In the undulatory theory of 

 light, the vibrations are supposed to be at right angles to the line of 

 propagation, or transverse. A complete oscillation to and fro is called 

 a sound-wave. 



It is evident that vibrating bodies may be made to perform and im- 

 part to the atmosphere oscillations of greater or less amplitude. The 

 intensity of sound is in proportion to the amplitude of the vibrations. 

 In a vibrating body capable of producing a definite number of waves of 

 sound in a second, it is evident that the greater the amplitude of the 

 wave, the greater is the velocity of the particles thrown into vibration. 

 It has been ascertained that there is an invariable mathematical relation 

 between the intensity of sound, the velocity of the conducting particles 

 and the amplitude of the waves ; and this is expressed by the formula, 

 that the intensity is proportional to the square of the amplitude. It is 

 evident, also, that the intensity of sound is diminished by distance. 



