Transparency and Opacity of the Atmosphere. 375 



Holmes had improved upon Daboll ; and with two instruments of 

 Holmes, not of the third but of the first order, our experiments 

 were made. On the 19th of May, at 3 miles distance, they became 

 useless as a fog-signal; at a distance of 4 miles, with paddles 

 stopped and all on board quiet, they w T ere wholly unheard. At a 

 distance of two miles from the Foreland the whistles tested on 

 May 19 became useless. The 12-o'clock gun, fired with a 1-lb. 

 charge at Drop Fort in Dover, was well heard on May 19, when 

 the horns and whistles were inaudible. On the 20th of May the 

 permeability of the atmosphere had somewhat increased, but the 

 steam-whistle failed to pierce it to a depth of 3 miles. At 4 miles 

 the horns, though aided by quietness on board, were barely heard. 

 By careful nursing, if I may use the expression, the horn-sounds 

 were carried to a distance of 6 miles. The superiority of the 18- 

 pounder gun, already employed by the Trinity House as a fog-signal, 

 over horns and whistles was on this day so decided as almost to 

 warrant its recommendation to the exclusion of all the other signals. 



Nothing occurred on the 2nd of June to exalt our hopes of the 

 trumpets and whistles. The horns were scarcely heard at a dis- 

 tance of 3 miles ; sometimes indeed they failed to be heard at 2 

 miles. By keeping every thing quiet on board, they were after- 

 wards heard to a distance of 6 miles — a result, however, mainly due 

 to the improved condition of the atmosphere. Considering the 

 demands as to sound-range made by writers on this subject, the de- 

 monstrated incompetence of horns and whistles of great reputed 

 power to meet these demands was not encouraging. On the 3rd of 

 June the atmosphere had changed surprisingly. It was loaded 

 overhead with dark and threatening clouds ; the sounds, neverthe- 

 less, were well heard beyond 9 miles. On June 10 the acoustic 

 transparency of the air was also very fair, the distance penetrated 

 being upwards of 8| miles. The subsidence of the sound near the 

 boundary of the acoustic shadow on the Dover side of the Foreland, 

 and before entering the shadow, was to-day sudden and extraordi- 

 nary, affecting equally both horns and guns. We were warned on 

 June 3 that the supremacy of the gun on one occasion by no means 

 implied its supremacy on all occasions, the self-same guns which 

 on May 20 had so far transcended the horns being on this day their 

 equals and nothing more. The 11th of June was employed in mas- 

 tering still further the facts relating to the subsidence of the sound 

 east and west of the Foreland — the cause of this subsidence being 

 in part due to the weakening of the sonorous waves by their divei'- 

 gence into the sound-shadow, and in part to interference. 



The atmosphere on the 25th of June was again very defective 

 acoustically. The sounds reached a maximum distance of 6| miles ; 

 but at 4 miles, on returning from the maximum distance, they were 

 very faint. The day had, as it were, darkened acoustically. On 

 this day the guns lost still further their preeminence, and at 5J 

 miles were inferior to the horn. ~No sounds whatever reached 

 Dover Pier on the 11th ; and it was only towards the close of the 

 day that they succeeded in reaching it on the 25th. Thus by slow 



