ACOUSTICS, 191 



Sound, therefore, cannot be propagated through a vacuum. 



"The loudest sound on earth, therefore, cannot penetrate beyond the 

 limits of our atmosphere ; and in the same manner, not the faintest sound 

 can reach our earth from any of the other planets. Thus the most fearful 

 explosions might take place in the moon, without our hearing anything of 

 them." 



How does the 410. The powei of air to transmit sound 

 ^unT''in°°air^ varies with its uniformity, its density, and its 

 ^y^ humidity. 



Whatever tends to agitate or disturb the condition of the atmosphere, affecta 

 the transmission of sounds. "When a strong wind blows from the hearer to- 

 ward a sounding body, a sound often ceases to be heard which would be 

 audible in a calm. Falling rain, or snow, interferes with the undulations of 

 sound-waves, and obstructs the transmission of sound. 



The fact that we hear sounds with greater distinctness by 

 ^I ^Bounds ^o^it than by day, may be, in part, accounted for by the cir- 

 more distinctly cumstance, that the different layers or strata of the atmosphero 

 by day?* '''*° ^^^ 1*^^ ^i^^^® ^^ variations in density and to currents, caused 

 by changes of temperature, at night than by day. The air at 

 night is also more still, from the suspension of business and hum of men. 

 Many sounds become perceptible during the night, which during the day are 

 completely stifled, before they reach the ear, by the din and discordant noises 

 of labor, business, and pleasure. 



Sound of any kind is transmitted to a greater distance in cold and clear 

 weather than in warm weather, the density of air being increased by cold 

 and diminshed by heat. 



On the top of high mountains, where the air is greatly rare- 

 mat are a- ggj ^Yie sound of the human voice can be heard for a short 

 lustrations of ' . 



the variation distance only ; and on the top of Mont Blanc, the explosion 

 of^^sound in ^f ^ pj^^^j appears no louder than that of a small cracker. 

 When persons descend to any considerable depth in a diving- 

 bell, the air around them is compressed by the weight of a considerable column 

 of water above them. In such circumstances, a whisper is almost as loud as a 

 shout in the open air; and when one speaks with ordinary force it produces 

 an effect so loud as to be painfuL 



Is air neces- 411. Air is Dot Deccssary to the production 

 production '^of ©^ sound, although most sounds are transmitted 

 sound? i^y j^g vibrations. Sound can he produced un- 



der water, and all bodies are more or less fitted, not only 

 to produce, hut also to transmit soimds. 

 whatsuhstan- 412. Sound is communicated more rapidly 

 ^10*"°°'^^ ^D^ more distinctly through solid bodies than 

 mostreaduy? through either liquids or gases. It is trans- 



