1874.] Transmission of Sound by the Atmosphere. 59 



tinued to the end of November. It also includes observations made 

 during the dense fog which enveloped London on December 9 and the 

 succeeding days. 



Gongs and bells were excluded from this investigation, in consequence 

 of their proved inferiority to other instruments of signalling. The experi- 

 ments were made with trumpets blown by powerfully compressed air, 

 with steam-whistles, guns, and a steam syren, associated with a trumpet 

 16 feet long. 



Daboll's horn, or trumpet, had been highly spoken of by writers on 

 fog-signals. A third-order apparatus of the kind had been reported as 

 sending its sound to a distance of from 7 to 9 miles against the wind, 

 and to a distance of 12 to 14 miles with the wind. 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 were wholly 

 unheard. At a distance of 2 miles from the Foreland the whistles tested 

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

 pression, 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 



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

 and whistles. The horns were scarcely heard at a distance of 3 miles ; 

 sometimes indeed they failed to be heard at 2 miles. By keeping every 

 thing quiet on board, they were afterwards 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 demonstrated 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, nevertheless, 

 were well heard beyond 9 miles. On June 10th 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 extraordinary, 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, 



