TRANSMISSION OF SOUND BY THE ATMOSPHERE. 
183 
towards us, fell suddenly and considerably in power. Before 
reaching Dover Pier it had ceased to be heard. The wind was 
here against the sound ; but this, though it contributed to the 
effect, could not account for it, nor could the proximity of the 
shadow account for it. To these two causes must have been 
added an acoustically flocculent though optically transparent 
atmosphere. The experiment demonstrates conclusively that 
there are atmospheric and local conditions which, when com- 
bined, 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 wind- 
ward was compared 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 windward. The 
whistles were tested this day in comparison with the syren de- 
prived 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 No- 
vember 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 atmosphere 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 un- 
questioning 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 
reasons 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, disappears, may be taken as a 
