Physics 54i 



connection with the Lighthouse service in England and by 

 reason of his own experiments on the absorption of sound. 

 A very decided difference of opinion was developed at a later 

 date between these two distinguished physicists, Professor 

 Henry disagreeing with Tyndall in the matter of the influ- 

 ence of fog, rain, snow, hail, etc., on the audibility of sounds. 

 Tyndall attributed many of the observed abnormal phenomena 

 " to the existence of acoustic clouds, consisting of portions of 

 the atmosphere in a flocculent or mottled condition, due to 

 the unequal distribution of heat and moisture which, absorb- 

 ing and reflecting the sound, produce an atmosphere of 

 acoustic opacity." Henry, on the contrary, while not deny- 

 ing the possible existence of such a condition, was inclined to 

 attribute such phenomena to the effect of the wind, in accord 

 with the hypothesis of Stokes, referred to in the remarks on 

 the paper by Doctor Taylor. The discussion growing out of 

 this difference of views was of such a nature, unfortunately, 

 as to give rise to some irritation, at least among the friends of 

 the two distinguished physicists, but it is generally believed 

 that subsequent observations have tended rather to confirm 

 the position taken by Henry. 



Among the valuable practical results of these researches in 

 sound the development of the use of the siren as an instru- 

 ment for producing sounds of great intensity must be men- 

 tioned. 



In his last report to the Lighthouse Board, not long 

 before his death, Henry summarized the results of all experi- 

 ments conducted by the Board up to that time, a few of the 

 more important conclusions being as follows : The audibility 

 of a sound at a distance (the state of the atmosphere being 

 constant) depends on the character of the sound ; to secure 

 audibility at a distance, the pitch of the sound should be 

 "medium"; the loudness, depending on the amplitude of vibra- 



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