66 Prof. J. Tyndall on the [Jan. 15, 



proximity of the shadow account for it. To these two causes must have 

 been added an acoustically flocculent though optically transparent atmo- 

 sphere. The experiment demonstrates conclusively that there are atmo- 

 spheric and local conditions which, when combined, prevent our most 

 powerful instruments from making more than a distant approach to the 

 performance which writers on fog-signals have demanded of them. 



On November 24 the sound of the syren pointed to windward was com- 

 pared at equal distances in front of and behind the instrument. It was 

 louder to leeward in the rear, than at equal distances to windward in front. 

 Hence, in a wind, the desirability of pointing the instrument to wind- 

 ward. The whistles were tested this day in comparison with the syren 

 deprived of its trumpet. The Canadian and the 8-inch whistles proved 

 the most effective; but the naked syren was as well heard as either of 

 them. As regards opacity, the 25th of November almost rivalled the 

 3rd of July. The gun failed to be heard at a distance of 2-8 miles, and 

 it yielded only a faint crack at 2| miles. 



Meanwhile this investigation has given us a knowledge of the atmo- 

 sphere in its relation to sound, of which no notion had been previously 

 entertained. While the velocity of sound has been the subject of refined 

 and repeated experiments, I am not aware that since the publication of a 

 celebrated paper by Dr. Derham, in the Philosophical Transactions for 

 1708, any systematic inquiry has been made into the causes which affect 

 the intensity of sound in the atmosphere. Derham's results, though 

 obtained at a time when the means of investigation were very defective, 

 have apparently been accepted with unquestioning trust by all subsequent 

 writers a fact which is, I think, in some part to be ascribed to the a 

 priori probability of his conclusions. 



Thus Dr. Robinson, relying apparently upon Derham, says, " Fog is a 

 powerful damper of sound," and he gives us physical reason why it must 

 be so. " It 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." And he adds further on, " The remarkable power 

 of fogs to deaden the report of guns has been often noticed." 



Assuming it, moreover, as probable that the measure of " a fog's power 

 in stopping sound " bears some simple relation to its opacity for light, 

 Dr. Robinson, adopting a suggestion of Mr. Alexander Cunningham, 

 states that " the distance at which a given object, say a flag or pole, dis- 

 appears, may be taken as a measure of the fog's power " to obstruct the 

 sound. This is quite in accordance with prevalent notions ; and granting 

 that the sound is dissipated, as assumed, by reflection from the particles 

 of fog, the conclusion follows that the greater the number of the reflecting 

 particles, the greater will be the waste of sound. But the number of 

 particles, or, in other words, the density of the fog, is declared by its 

 action upon light ; hence the optical opacity will be a measure of the 

 acoustic opacity. 





