44 Curiosities of Sound. 



pump and bell experiment, also dependent upon the density of 

 the air in which it is generated, growing feebler as that density 

 is reduced. Professor Tyndall says, ' c Supposing the summit of 

 Mont Blanc to be equally distant from the top of the Aiguille 

 Verte and the Bridge of Chamouni, and supposing two 

 observers stationed, the one upon the bridge, and the other 

 upon the Aiguille, the sound of a cannon fired on Mont Blanc 

 would reach both observers with the same intensity, though in 

 the one case the sound would pursue its way through the rare 

 air above, while in the other it would descend through the rare 

 air below." If the cannon were fired in the dense air of the 

 bridge, its sound would reach the top of the mountain ; while 

 if fired in the thin air of the mountain-top, it might be too 

 weak to be heard below. 



Sound grows weaker by spreading. When strong vibra- 

 tions act upon a small number of air particles, they throw them 

 into violent commotion ; but when the same amount of force 

 operates on a much larger quantity of air, the movement of 

 each particle is less rapid, and the sound declines. By speaking 

 through pipes we check the lateral propagation of the sound- 

 wave, and hence the voice can be heard at great distances ; 

 and it is easy, by means of apparatus now common in large 

 places of business, to hold conversations with persons in any 

 part of extensive premises. It is a curious instance of the 

 force foolishly allowed to conventional habits, that this mode of 

 communication is not applied in ordinary domestic life. It 

 would be more rational for the parlour to adopt this quick 

 mode of conveying its orders to the kitchen, than to ring bells 

 to summon domestics to hear what is wanted, and then go and 

 fetch it — a proceeding involving a waste of labour and a loss of 

 time. Barbaric ideas of grandeur always include waste ; but as 

 civilization advances, it will be seen that the most dignified 

 thing is to act in the most rational way, and to accomplish 

 desirable objects with the greatest economy of any sort of 

 force. 



Sound may be readily deadened by the interposition of 

 non-elastic bodies. It would be difficult to talk through a 

 feather bed, though easy to be heard through the wainscot 

 panelling of a room. 



Sound is capable of being reflected like light. Echoes 

 are the result of natural or artificial arrangements, which send 

 the waves back again, as many times as the echo repeats, and 

 by interposing a balloon filled with cai-bonic acid in the way 

 of sound-waves, they may be bent out of their diverging 

 course, and concentrated just as a spherical lens concentrates 

 beams of light. A parabolic reflector will send the rays of a 

 lamp in a long narrow divergent cone for many miles across 



