ON THE MOTIONS OF SOUND. 423 



the signal at Port Elizabeth is always heard at Portland, a distance 

 of nine miles. The observations at the South Foreland, where the 

 sound has been proved to reach a distance of more than twelve miles 

 against the wind, backed by decisive experiments, reduce to certainty 

 the surmises of General Duane. It has, for example, been proved that 

 a couple of gas-flames placed in a chamber can, in a minute or two, 

 render its air so non-homogeneous as to cut a sound practically off; 

 while the same sound passes without sensible impediment through 

 showers 'of paper-scraps, seeds, bran, rain-drops, and through fumes 

 and fogs of the densest description. The sound also passes through 

 thick layers of calico, silk, serge, flannel, baize, close felt, and through 

 pads of cotton-net impervious to the strongest light. 



As long indeed as the air on which snow, hail, rain, or fog, is 

 suspended is homogeneous, so long will sound pass through the air, 

 sensibly heedless of the suspended matter.* This point is illustrated 

 upon a large scale by my own observations on the Mer de Glace, and 

 by those of General Duane, at Portland, which prove the snow-laden 

 air from the northeast to be a highly homogeneous medium. Prof. 

 Henry thus accounts for the fact that the northeast snow-wind renders 

 tlie sound of Cape Elizabeth audible at Portland : In the higher 

 regions of the atmosjDhere he places an ideal wind, blowing in a 

 direction opposed to the real one, which always accompanies the 

 latter, and which more than neutralizes its action. In speculating 

 thus he bases himself on the reasoning of Prof. Stokes, according to 

 which a sound-wave moving against the wind is tilted upward. The 

 upper and opposing wind is invented for the purpose of tilting again 

 the already lifted sound-wave downward. Prof. Henry does not ex- 

 plain how the sound-wave recrosses the hostile lower current, nor does 

 he give any definite notion of the conditions under which it can be 

 shown that it will reach the observer. 



This, so far as I know, is the only theoretic gleam cast by the 

 Washington Report on the conflicting results w^hich have hitherto 

 rendered experiments on fog-signals so bewildering. I fear it is an 

 ignis fatuus, instead of a safe guiding light. Prof. Henry, however, 

 boldly applies the hypothesis in a variety of instances. But he dwells 

 with particular emphasis upon a case of non-reciprocity -which he con- 

 siders absolutely fatal to my views regarding the flocculence of the 

 atmosphere. The observation was made on board the steamer City 

 of Richmond, during a thick fog in a night of 1872. " The vessel was 

 approaching Whitehead from the southwestward, when, at a distance 

 of about six miles from the station, the fog-signal, which is a ten-inch 

 steam-whistle, was distinctly perceived, and continued to be heard 

 with increasing intensity of sound until within about three miles, 

 when the sound suddenly ceased to be heard, and was not perceived 

 again until the vessel, approached within a quarter of a mile of the sta- 



' This is not more surprising than the passage of radiant heat through rock-salt. 



