194 



WELLS'S NATURAL PHILOSOPHY. 



Fig. 187, ^^'^ * ^ ^^^ '^ f^i ^'^S- 1^7, represent two se- 



ries of sound undulations, advancing in such 

 a manner as to cause the elevation of one at e 

 to correspond with the depression of the other 

 at /; then if both are equal in intensity, they 

 will neutralize each other, and an instant of si- 

 lence will be produced. This fact may be very 

 prettily illustrated by holding a common tuning- 

 fork, after it has been put in vibration, over tho 

 mouth of a cylindrical glass vessel, as A, Fig. 188. The air contained witliin 



Fig. 188. 



B 



the vessel will assume sonorous vibrations, and a 

 tone will be produced. If now a second glass 

 cylinder be held in the position B, at right angles 

 to A, the musical tone previously heard will cease ; 

 but if either cylinder be removed, the sound will 

 be renewed again in the other. In this curious 

 experiment, the silence arises from the interference 

 ^^ _^ of the two sounds. 



Another example of this phenomena may bo 

 produced by the tuning-fork alone. If this instrument, after being put into vi- 

 bration, bo held at a great distance from tho ear, and slowly turned round its 

 axis, a position of the two branches will be found at which the sound will 

 become inaudible. This position will correspond to the points of interference 

 of the two systems of undulations propagated from the two branches, or 

 prongs of the fork. 



uponwhatdoes 417. The loudness of a sound, or its degree 

 l^" iounT'^de- 0^ intensity, depends on tlie force -witli wliieh 



Eound 

 pend? 



the vibrations of a sounding body are made. 



SECTION I 



What are mn. 

 eical sounds ? 



MUSICAL SOUNDS. 



418. All vibrations of sonorous bodies which 

 are uniform, regular, and sufficiently rapid, 

 produce agreeable, or musical sounds, 

 whatisthedif- 419. What constitutes the particular differ- 

 amusTcarsMmd ^^^0 bctween a noise and a musical sound is 

 and a noise? ^^^ certainly known. A noise, however, is 

 supposed to be occasioned by impulses communicated ir- 

 regularly to the ear ; but in a musical sound the vibra- 

 tions of the sonorous body, and consequently the undula- 

 tions of the air, must be all exactlv similar in duration 



