542 ' THE POPULAR SCIENCE MONTHLY. 



" Nearly all that is known about fog-signals is to be found in tbe ' Report 

 on Lights and Beacons ; ' and of it much is little better than conjecture. Its 

 substance is as follows : 



" ' Light is scarcely available for this purpose. Blue lights are used in the 

 Iloogly; but it is uot stated at what distance they are visible in fog; their 

 glare may be seen farther than their flame. 1 It might, however, be desirable to 

 ascertain how far the electric light or its flash can be traced. 2 



" ' Sound is the only known means really effective ; but about it testimonies 

 are conflicting, and there is scarcely one fact relating to its use as a signal 

 which can be considered as established. Even the most important of all, the 

 distance at which it ceases to be heard, is undecided. 



" ' Up to the present time all signal-sounds have been made in air, though 

 this medium has grave disadvantages: its own currents interfere with the 

 sound-waves, so that a gun or bell which is heard several miles down the wind 

 is inaudible more than a few furlongs up it. A still greater evil is that it is 

 least effective when most needed ; for fog is a powerful damper of sound.' " 



Dr. Robinson here expresses the universally prevalent opinion, and 

 he then assigns the theoretic cause. Fog, he says, " is a mixture of air 

 and globules of water, and, at each of the innumerable surfaces where 

 these two touch, a portion of the vibration is reflected and lost. 3 . . . 

 Snow produces a similar effect, and one still more injurious." 



Reflection being thus considered to take place at the surfaces of 

 the suspended particles, it followed that the greater the number of 

 particles, or, in other words, the denser the fog, the more injurious 

 would be its action upon sound. Hence optic transparency came to 

 be considered a measure of acoustic transparency. On this point Dr. 

 Robinson, in the letter referred to, expresses himself thus : " At the 

 outset, it is obvious that, to make experiments comparable, we must 

 have some measure of the fog's power of stopping sound, without at- 

 tending to which the most anomalous results may be expected. It 

 seems probable that this will bear some simple relation to its opacity 

 to light, and that the distance at which a given object, as a flag or 

 pole, disappears, may be taken as the measure. . . . Still, clear air" 

 was regarded in this letter as the best vehicle of sound, the alleged 

 action of fogs, rain, and snow, being ascribed to their rendering the 

 atmosphere " a discontinuous medium." 



Prior to this investigation the views here enunciated were those 

 universally entertained. That sound is unable to penetrate fogs was 

 taken to be " a matter of common observation." The bells and horns 

 of ships were affirmed "not to be heard so far in fogs as in clear 

 weather." In the fogs of London, the noise of the carriage-wheels 

 was reported to be so much diminished that " they seem to be at a 

 distance when really close by." My knowledge does not inform me 

 of the existence of any other source for these opinions regarding the 



1 A very sagacious remark, as observation proves. 



4 Powerful electric lights have been since established, and found ineffectual. 

 3 This is also Sir John Herschel's way of regarding the subject, " Essay on Sound," 

 par. 38. 



