46 SECTION PHYSICS. 



shawl, and you see that it has hardly any effect upon the waves of 

 sound, they go through it as if it were not there. I have here a piece 

 of close felt, impervious to the noonday sun ; it has hardly any effect 

 upon the waves of sound. They go through it almost as if it were 

 not there. Here are a hundred layers of cotton net all sewn together, 

 and you will see it has no sensible effect upon the waves of sound. 

 Here is another hundred which I add to them, and you see the thing is 

 sensibly transparent to the waves of sound. A piece of card-board, 

 on the contrary, immediately cuts off the sound. A piece of oilskin, 

 also, stops the waves of sound. Here is a piece of cambric, which 

 is really like nothing as regards its action upon the sound waves. 

 But the whole secret of it is, that these waves of sound possess the 

 power in a most astonishing manner of getting through any substance 

 in whose interstices the air is continuous. If you can suck the air 

 freely through a body, the waves of sound will go through it. This 

 piece of cambric, for example, has no sensible effect upon the waves 

 of sound. I dip it into water, so as to cause it to be thoroughly wetted 

 by the water, and when that is done you will find that its perviousness 

 to sound ceases. We have here a film of water filling up the inter- 

 stices of the cambric, and the result is that the sound waves are 

 entirely cut off. Let us repeat this experiment the converse way. I 

 put this cambric between bibulous paper and rub it, so as to take 

 away the water from the interstices, and you see its transparency to 

 sound is restored. I dip it again, and then it stops the sound. There- 

 fore, as long as there is no solution of continuity in the air, the waves 

 of sound possess this extraordinary power of permeation. You have 

 seen the action of these layers of differently heated air upon the waves 

 of sound. Sometimes the whole atmosphere is filled with these layers 

 of air of different densities produced in part by heat and in part by 

 different degrees of saturation ot aqueous vapour, so that you have 

 here what we may fairly call an acoustic fog in the air, uniformly 

 diffused through it. You can go beyond that. These masses of non- 

 homogeneous air sometimes drift through the air with definite bound- 

 aries, just as the clouds of the ordinary atmosphere drift over the 

 blue sky; and if you simply exercise the patience of observing a 

 bell or a clock strike with a certain definite force for a single week, 

 sometimes for a single day, you are able to see with the mind's eye, 



