554 THE POPULAR SCIENCE MONTHLY. 



or the capability of the conflicting witnesses, for the variations of the 

 atmosphere are more than sufficient to account for theirs. The mis- 

 take, indeed, hitherto has been, not in reporting incorrectly, but in 

 neglecting the monotonous operation of repeating the observations 

 during a sufficient time. I shall have occasion to remark subsequent- 

 ly on the mischief likely to arise from giving instruction to mariners 

 founded on observations of this incomplete character. 



It required, however, long pondering and repeated observation be- 

 fore this conclusion took firm root in my mind ; for it was opposed to 

 the results of great observers, and to the statements of celebrated 

 writers. In science, as elsewhere, a mind of any depth, which accepts 

 a doctrine undoubtingly, discards it unwillingly. The question of 

 aerial echoes has an historic interest. While cloud-echoes have been 

 accepted as demonstrated by observation, it has been hitherto held as 

 established that audible echoes never occur in optically clear air. "We 

 owe this opinion to the admirable report of Arago on the experiments 

 made to determine the velocity of sound at Montlhery and Villejuif 

 in 1822. 1 Arago's account of the phenomenon observed by him and 

 and his colleagues is as follows : " Before ending this note we will 

 only add that the shots fired at Montlhery were accompanied by a 

 rumbling like that of thunder, which lasted from 20 to 25 seconds. 

 Nothing of this kind occurred at Villejuif. Once we heard two dis- 

 tinct reports, a second apart, of the Montlhery cannon. In two other 

 cases the report of the same gun was followed by a prolonged rum- 

 bling. These phenomena never occurred without clouds. Under a 

 clear sky the sounds were single and instantaneous. May we not, 

 therefore, conclude that the multiple reports of the Montlhery gun 

 heard at Villejuif were echoes from the clouds, and may we not 

 accept this fact as favorable to the explanation given by certain phys- 

 icists of the rolling of thunder?" 



I think both the fact and the inference need reconsideration. For 

 our observations prove to demonstration that air of perfect visual trans- 

 parency is competent to produce echoes of great intensity and long 

 duration. The subject is worthy of additional illustration. On the 



1 Sir John Herschel gives the following account of Arago's observation : " The 

 rolling of thunder has been attributed to echoes among the clouds ; and, if it is con- 

 sidered that a cloud is a collection of particles of water, however minute, in a liquid 

 state, and therefore each individually capable of reflecting sound, there is no reason 

 why very loud sounds should not be reverberated confusedly (like bright lights) from 

 a cloud. And that such is the case has been ascertained by direct observation on 

 the sound of cannon. Messrs. Arago, Matthieu, and Prony, in their experiments on 

 the velocity of sound, observed that under a perfectly clear sky the explosions of their 

 guns were always single and sharp ; whereas, when the sky was overcast, and even when 

 a cloud came in sight over any considerable part of the horizon, they were frequently 

 accompanied by a long-continued roll like thunder." (" Essay on Sound," par. 38.) The 

 distant clouds would imply a long interval between sound and echo, but nothing of the 

 kind is reported. 



