Y38 SPECIAL SENSES. 



fifth and octave, the result is a major chord, the sound of which is very dif- 

 ferent from that of a single note or of a note with its octave. If the third 

 be diminished by a semitone, there is a different quality, which is peculiar to 

 minor chords. In this way a great variety of musical sounds may be made 

 upon a single instrument, as the piano ; and by the harmonious combinations 

 of the notes of different instruments and of different registers of the human 

 voice, as in choral and orchestral compositions, shades of effect, almost in- 

 numerable, may be produced. The modification of sounds in this way con- 

 stitutes 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 completeness is 

 attained. Different chords of 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. 



Laivs 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 care- 

 fully arranged in vacuo. Although the stroke and the vibration can readily 

 be seen, there is no sound ; and if air be gradually 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 conduction and are said to be longitudinal. In 

 the uridulatory theory of light, the vibrations are supposed to be at right 

 angles to the line of propagation, or transversal. A complete oscillation to 

 and fro is called a sound-wave. 



It is evident that vibrating bodies may be made to perform and impart 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. The sound, as the waves recede from the sonorous 



