LAWS OF SONOROUS VIBRATIONS. 739 



body, becomes distributed over an increased area. The propagation of sound 

 has been reduced also to the formula, that the intensity diminishes in pro- 

 portion to the square of the distance. 



Sonorous vibrations are subject to many of the laws of reflection of light. 

 Sound may be absorbed by soft and non-vibrating surfaces, in the same way 

 that certain surfaces aborb the rays of light. By carefully arranged convex 

 surfaces, the waves of sound may readily be collected to a focus. These laws 

 of the reflection of sonorous waves explain echoes and the conduction of 

 sound by confined strata of air, as in tubes. To make the parallel between 

 sonorous and luminous transmission more complete, it has been ascertained 

 that the waves of sound may be refracted to a focus, by being made to pass 

 through an acoustic lens, as a balloon filled with carbon dioxide. The waves 

 of sound may also be deflected around solid bodies, when they produce what 

 have been called by Tyndall, shadows of sound. 



Any one observing the sound produced by the blow of an axe can note 

 the fact that sound is transmitted with much less rapidity than light. At a 

 short distance the view of the body is practically instantaneous ; but there is 

 a considerable interval between the blow and the sound. This interval re- 

 presents the velocity of sonorous conduction. This fact is also illustrated by 

 the interval between a flash of lightning and the sound of thunder. The 

 velocity of sound depends upon the density and elasticity of the conducting 

 medium. The rate of conduction of sound, by atmospheric air at the freezing- 

 point of water, is about 1,090 feet (332 metres) per second. This rate pre- 

 sents comparatively slight variations for the different gases, but it is very 

 much more rapid in liquids and in solids. , 



Noise and Musical Sounds. There is a well defined physical as well as 

 an aesthetic distinction between noise and music. Taking as examples, sin- 

 gle sounds, a sound becomes noise when the air is thrown into confused and 

 irregular vibrations. A noise may be composed of musical sounds, when 

 these are not in accord with each other, and sounds called musical are not 

 always entirely free from discordant vibrations. A noise possesses intensity, 

 varying with the amplitude of the vibrations, and it may have different 

 qualities depending upon the form of its vibrations. A noise may be called 

 dull, sharp, ringing, metallic, hollow etc., these terms expressing qualities 

 that are readily understood. A noise may also be called sharp or low in 

 pitch, as the rapid or slow vibrations predominate, without answering the 

 requirements of musical sounds. 



A musical sound consists of vibrations following each other at regular in- 

 tervals, provided that the succession of waves be not too slow or too rapid. 

 When the vibrations are too slow, there is an appreciable succession of im- 

 pulses, and the sound is not musical. When they are too rapid, the sound 

 is excessively sharp, but it is painfully acute and has no pitch that can be ac- 

 curately determined by the auditory apparatus. Such sounds may be occa- 

 sionally employed in musical compositions, but in themselves they are not 

 strictly musical. 



Musical sounds have the characters of duration, intensity, pitch and 



