THE ATMOSPHERE AND FOG-SIGNALING. 6 97 



passed through pure air. The first action was manifestly due to dif- 

 ferences of temperature, and disappeared when the temperature was 

 equalized. 



The cupboard was next filled with the dense fumes of gunpowder. 

 At first there was a slight action ; but this disappeared even more 

 rapidly than in the case of the phosphorus, the sound passing as if 

 no fumes were there. It required less than half a minute to abolish 

 the action in the case of the phosphorus, but a few seconds sufficed 

 in the case of the gunpowder. The fumes were far more than sufficient 

 to quench the candle-flame. 



The dense smoke of resin, when the temperature had become equa- 

 ble, exerted no action on the sound. 



The fumes of gum-mastic were equally ineffectual. 



The fumes of the perchloride of tin, though of extraordinary den- 

 sity, exerted no sensible effect upon the sound. 



Exceedingly dense fumes of chloride of ammonium next filled the 

 cupboard. A fraction of the length of the three-foot tube sufficed to 

 quench the candle-flame. Soon after the cupboard was filled, the 

 sound passed without the least sensible deterioration. An aperture 

 at the top of the cupboard was opened ; but, though a dense smoke- 

 column ascended through it, many minutes elapsed before the candle- 

 flame could be seen through the attenuated fog. 



Steam from a co])per boiler was so copiously admitted into the 

 cupboard as to fill it with a dense cloud. No real cloud was ever 

 so dense ; still the sound passed through it without the least sen- 

 sible diminution. This being the case, cloud-echoes are not a likely 

 phenomenon. 



In all of these cases, when a couple of Bunsen's burners were ignited 

 within the cupboard containing the fumes, less than a minute's action 

 rendered the air so heterogeneous that the sensitive flame was com- 

 pletely stilled. 



These acoustically inactive fogs were subsequently proved com- 

 petent to cut off the electric light. 



Experiment and observation go, therefore, hand-in-hand in demon- 

 strating that fogs have no sensible action upon sound ; the notion of 

 their impenetrability which so powerfully retarded the introduction 

 of phonic coast-signals being thus abolished, we have solid ground 

 for the hope that disasters due to fogs and thick weather will, in the 

 future, be materially mitigated. 



Action of Wind. In stormy weather we were frequently for- 

 saken by our steamer, which had to seek shelter in the Downs or Mar- 

 gate Roads, and on such occasions the opportunity was turned to ac- 

 count to determine the effect of the wind. On October 11th, accom- 

 panied by Mr. Douglas and Mr. Edwards, I walked along the cliffs 

 to Dover Castle toward the Foreland, the wind blowing strongly 



