150 Royal Society : — 



significance), the transmitted sound reached its maximum range, 

 the gun sounds being heard at the Quenocs buoy, which is 16| 

 nautical miles from the South Foreland. I have already stated that 

 the duration of the air-echoes indicates " the atmospheric depths " 

 from which they come*. An optical analogy may help us here. 

 Let light fall upon chalk, the light is wholly scattered by the 

 superficial particles ; let the chalk be powdered and mixed with 

 water, light reaches the observer from a far greater depth of the 

 turbid liquid. The solid chalk typifies the action of exceedingly 

 dense acoustic clouds ; the chalk and water that of clouds of mode- 

 rate density. In the one case we have echoes of short, in the other 

 echoes of long duration. These considerations prepare us for the 

 inference that Montlhery, on the occasion referred to, must have 

 been surrounded by a highly diacoustic atmosphere ; while the 

 shortness of the echoes at Villejuif shows the atmosphere sur- 

 rounding that station to have been acoustically opaque. 



Have we any clue to the cause of the opacity ? I think we 

 have. Villejuif is close to Paris, and over it, with the observed 

 light wind, was slowly wafted the air from the city. Thousands of 

 chimneys to windward of Villejuif were discharging their heated 

 currents ; so that an atmosphere non-homogeneous in a high degree 

 must have surrounded that station. At no great height in the 

 atmosphere the equilibrium of temperature would be established. 

 The non-homogeneous air surrounding Villejuif is experimentally 

 typified by our screen with the source of sound close behind it, 

 the upper edge of the screen representing the place where equi- 

 librium of temperature was established in the atmosphere above 

 the station. In virtue of its proximity to the screen, the echoes 

 from our sounding-reed would, in the case here supposed, so blend 

 with the direct sound as to be practically indistinguishable from it, 

 as the echoes at Villejuif followed the direct sound so hotly, and 

 vanished so rapidly, that they escaped observation. And as our 

 sensitive flame, at a distance, failed to be affected by the sounding 

 body placed close behind the card-board screen, so, I take it, did 

 the observers at Montlhery fail to hear the sounds of the Villejuif 

 gun. This is the explanation of Arago's difficulty which I have 

 the honour to submit to the Royal Society. 



Something further may be done towards the experimental eluci- 

 dation of this subject. The facility with which sounds pass through 

 textile fabrics has been already illustrated t, a layer of cambric or 

 calico, or even of thick flannel or baize, being found competent to 

 intercept but a fraction of the sound from a vibrating reed. Such 

 a layer of calico may be taken to represent a layer of air differ- 

 entiated from its neighbours by temperature or moisture ; while a 

 succession of such sheets of calico may be taken to represent suc- 

 cessive layers of non-homogeneous air. 



* Phil. Trans. 1874, pt. i. p. 202. 

 t Phil. Trans. 1874, pt, i. p. 208. 



