passed through pure air. The first action was manifestly due to differences 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 equable, exerted no action on the sound.
The fumes of gum-mastic were equally ineffectual.
The fumes of the perchloride of tin, though of extraordinary density, 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 copper 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 sensible 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 completely stilled.
These acoustically inactive fogs were subsequently proved competent to cut off the electric light.
Experiment and observation go, therefore, hand-in-hand in demonstrating 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 forsaken by our steamer, which had to seek shelter in the Downs or Margate Roads, and on such occasions the opportunity was turned to account to determine the effect of the wind. On October 11th, accompanied by Mr. Douglas and Mr. Edwards, I walked along the cliffs to Dover Castle toward the Foreland, the wind blowing strongly