SCIENCE OF COMMON THINGS. 117 



Solids transmit sound. The earth conducts sound. Sound vibrations. 



73S Why is this mixture of tin and copper used for bell-metal ? 



Because it is much harder and more elastic than any 

 of the pure metals. 



733 Are. solids capable of transmitting sounds f 



All solid bodies which possess elasticity have the 

 power of propagating or transmitting sounds. 



734 What easy experiment illustrates the transmission of sound by 

 solids f 



When a stick is held between the teeth at one ex- 

 tremity, and the other is placed in contact with a table, 

 the scratch of a pin on the table may be heard with 

 great distinctness, though both ears be stopped. 



735 Does tfie earth conduct sound f 



The earth often conducts sound, so as to render it 

 sensible to the ear, when the air fails to do so. It is 

 well known that the approach of a troop of horse can 

 be heard at a distance by putting the ear to the ground, 

 and savages practise this method of ascertaining the 

 approach of persons from a great distance. 



73Q What purpose is subserved by tJie body of a stringed instrument t 



The string of an instrument, when caused to vibrate, 

 communicates the vibrations to the matter composing 

 the body of the instrument and the surrounding air, 

 and thus a tone or musical note is produced and ren- 

 dered audible to the ear. 



737 How are aerial vibrations or pulses communicated? 



The air, encompassing sounding bodies on every side, 

 conveys the sensation of sound in all directions ; there- 

 fore the aerial vibrations, or, as they have been termed, 

 "pulses" must be communicated successively and ge- 

 nerally throughout the whole space within the limits 

 of which they are capable of affecting the ear. 



738 To what have the sound vibrations or pulsations been compared? 



To the waves spreading in concentric circles over 

 the smooth surface of water. 



"When a stone is thrown into water, the liquid waves are propagated 

 not only directly forward from the centre, but if they encounter any obr 

 struction, as from a floating body, they will bend their course round the 

 sides of the obstacle, and spread out obliquely beycm4 it. So th,e undu. 



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