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



the self-same guns which on May 20th had so far transcended the horns 

 being on this day their equals and nothing more. The llth of June was 

 employed in mastering 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 diver- 

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



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

 cally. 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 5| miles were inferior to the horn. 

 JSTo sounds whatever reached Dover Pier on the llth ; and it was only 

 towards the close of the day that they succeeded in reaching it on the 

 25th. Thus by slow degrees the caprices of the atmosphere made them- 

 selves 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, 

 underwent 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 J miles. The sounds were also heard at the Varne light-ship, 

 which is 12| miles from the Foreland. The atmosphere had become 

 decidedly clearer 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 12 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 atmosphere, 

 clearly showing that conclusions founded upon one day's observations 

 might utterly break down in the presence of the phenomena 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 



