376 Royal Institution ; — Prof. Tyndall on the Acoustic 



degrees the caprices of the atmosphere made themselves known to 

 us, showing us subsequently that within the limits of a single day, 

 even within a single minute, the air, as a vehicle of sound, under- 

 went most serious variations. The 26th of June was a far better 

 day than its predecessor, the acoustic range being over 9^ miles. 

 The direction of the wind was less favourable to the sound on this 

 day than on the preceding one, plainly proving that something else 

 than the wind must play an important part in shortening the sound- 

 range. 



On the 1st of July we experimented upon a rotating horn, and 

 heard its direct or axial blast, which proved to be the strongest, at a 

 distance of 10^- miles. The sounds were also heard at the Yarne 

 light-ship, which is 12 j miles from the Foreland. The atmosphere 

 had become decidedly clear acoustically, but not so optically ; for 

 on this day thick haze obscured the white cliffs of the Foreland. In 

 fact, on days of far greater optical purity, the sound had failed to 

 reach one third of the distance attained to-day. In the light of 

 such a fact, any attempt to make optical transparency a measure of 

 acoustic transparency must be seen to be delusive. On the 1st of 

 July a 12-inch American whistle, of which we had heard a highly 

 favourable account, was tried in place of the 12-inch English whistle ; 

 but, like its predecessor, the performance of the new instrument 

 fell behind that of the horns. An interval of twelve hours sufficed 

 to convert the acoustically clear atmosphere of July 1 into an 

 opaque one ; for on the 2nd of July even the horn-sounds, with 

 paddles stopped and all noiseless on board, could not penetrate 

 further than 4 miles. 



Thus each succeeding day provided us with a virtually new atmo- 

 sphere, clearly showing that conclusions founded upon one day's 

 observations might utterly break down in the presence of the phe- 

 nomena of another day. This was most impressively demonstrated 

 on the day now to be referred to. The acoustic imperviousness of 

 the 3rd of July was found to be still greater than that of the 2nd, 

 while the optical purity of the day was sensibly perfect. The cliffs 

 of the Foreland could be seen to-day at ten times the distance at 

 which they ceased to be visible on the 1st, while the sounds were 

 cut off at one sixth of the distance. At 2 p.m. neither guns nor 

 trumpets were able to pierce the transparent air to a depth of 3, 

 hardly to a depth of 2 miles. This extraordinary opacity was proved 

 conclusively to arise from the irregular admixture with the air of 

 the aqueous vapour raised by a powerful sun. This vapour, though 

 perfectly invisible, produced an acoustic cloud impervious, to the 

 sound, and from which the sound-waves were thrown back as the 

 waves of light are from an ordinary cloud. The waves thus refused 

 transmission produced by their reflection echoes of extraordinary 

 strength and duration. This, I may remark, is the first time that 

 audible echoes have been proved to be reflected from an optically 

 transparent atmosphere. By the lowering of the sun the produc- 

 tion of the vapour was checked, and the transmissive power of the 

 atmosphere restored to such an extent that, at a distance of 2 miles 



