18 4 ANNUAL RECORD OF SCIENCE AND INDUSTRY. 



and some other days to act as my amanuensis, at 1 P.M. I 

 was rowed to the shore, and landed at the base of the South 

 Foreland Cliff. The body of air which had already shown 

 such extraordinary power to intercept the sound, and which 

 manifested this power still more impressively later in the 

 day, was now in front of us. On it the sonorous waves im- 

 pinged, and from it they were sent back to us with astonish- 

 ing intensity. The instruments, hidden from view, were on 

 the summit of a cliff two hundred and thirty-five feet above 

 us, the sea was smooth and clear of ships, the atmosphere 

 was without a cloud, and there was no object in sight which 

 could possibly produce the observed effect. From the per- 

 fectly transparent air the echoes came, at first with a strength 

 apparently but little less than that of the direct sound, and 

 then dying gradually and continuously away. A remark 

 made by my talented companion in his note-book at the time 

 shows how the phenomenon affected him 'Beyond saying 

 that the echoes seemed to come from the expanse of ocean, 

 it did not appear possible to indicate any more definite point 

 of reflection.' Indeed, no such point was to be seen ; the 

 echoes reached us as if by magic, from absolutely invisible 

 walls. 



"Here, in my opinion, we have the key to many of the 

 mysteries and discrepancies of evidence which beset this 

 question. The foregoing observations show that there is no 

 need to doubt either the veracity or capability of the con- 

 flicting witnesses, for the variations of the atmosphere are 

 more than sufficient to account for theirs. The mistake, in- 

 deed, hitherto has been, not in reporting incorrectly, but in 

 neglecting the monotonous operation of repeating the ob- 

 servations during a sufficient time." 



The above explanation given by Tyndall has not been 

 generally received by men of science. Certainly his experi- 

 ments were not sufficiently extended and varied to enable one 

 to form a conclusive opinion. M. Baudrimont, in the Comptes 

 Rendus for April, 1874, calls attention to the improbability 

 of the existence of vertical columns or walls of dense and 

 rarefied air existing over the sea on a calm day. He is of 

 opinion that there would exist horizontal layers of air, grad- 

 ually increasing in density as we ascended; and as sound 

 progresses more rapidly in warm and moist air than in dry 



