REFLECTION OF SOUND. 47 



those invisible accoustic clouds drifting through the atmosphere just 

 as certainly as the clouds that you see floating before your eyes 

 in the blue heavens. Standing, for instance at the end of the 

 Serpentine, and listening to Big Ben, let it be twelve o'clock in the 

 day, or twelve at night, you hear the first stroke of the clock with 

 powerful force the second with power, the third absolutely quenched, 

 the fourth quenched, the fifth quenched, a feeble sound at the sixth, 

 the seventh comes out with sudden and extraordinary power, the eighth 

 with great power, the ninth and tenth are again silent ; and in this way 

 by observing a clock, or bell, struck with a definite mechanical power 

 you can realise to your minds the drifting of these acoustic clouds 

 through the atmosphere just as vividly as the clouds you see with your 

 eyes. Not only is this the case, but these accoustic clouds are also super- 

 posed upon ordinary clouds, and upon ordinary fog. You find this 

 action upon bells during foggy weather, the fog itself having no 

 sensible influence on the sound. Not by the most accurate measure- 

 ments can any sensible influence be established with regard to fog, 

 nor any sensible interception of the sound waves by the particles 

 of fog. To follow up the parallel a little further, you see a cloud 

 shone upon by the sun, and you sometimes receive dazzling white 

 light from the surfaces of that cloud ; those are the echoes, so to 

 say, of the light reflected back from the cloud. In precisely the 

 same way, with regard to these acoustic clouds, if you have your 

 source of sound strong enough, you get an echo of a most extra- 

 ordinary character. Put yourself in front of an acoustic cloud, and 

 you are able to detect by reflection the very sound that had been 

 refused transmission. I am sure you will find no difficulty in believing, 

 after having seen these experiments on bodies impervious to light, 

 but transmitting the waves of sound, that there is no connection 

 whatever between optical transparency and acoustical transparency. 

 What has been alleged regarding fog is also true of hail, rain, and 

 snow, and we find the same thing both by observation and by experi- 

 ment. I had a chamber at the Royal Institution, which I filled with 

 fumes, so dense that a layer of a couple of feet was sufficient entirely 

 to efface the strongest electric light. This fog was sometimes produced 

 by the combustion of gunpowder, sometimes by the combustion of 

 phosphorous, sometimes by the combustion of resin, and sometimes by 



