1874] WRITINGS OF JOSEPH HENRY. 427 



whistle of the steamer, which was blown as soon as the whistle 

 at the station ceased to be heard, in order to call the attention 

 of the keeper to what was supposed to be a neglect of his duty 

 in intermitting the operations of his signal. It should be ob- 

 served that the sound from the steamer in this case was pro- 

 duced by a 6-inch whistle, while that of the station was from 

 an instrument of the same kind, of 10 inches in diameter; or 

 in other words a lesser sound was heard from the steamer, 

 while a sound of greater volume from the station was unheard 

 in an opposite direction. It is evident that this result could 

 not be due to any mottled condition or want of acoustic trans- 

 parency of the atmosphere, since this would absorb the sound 

 equally in both directions. The only plausible explanation of 

 this phenomenon is that which refers it to the action of the 

 wind. In the case of the sound from the steamer, the wind 

 was favorable for its transmission, and hence it is not strange 

 that its sound should be heard on the island when the sound 

 from the other instrument could .not be heard on the steamer. 

 To explain on the same principle the fact of the hearing of 

 the sound at the distance of six miles, and afterward of 

 losing it at the distance of three miles, we have only to sup- 

 pose that in the first instance the retarding effect of the wind 

 was small, and that in the second it became much greater 

 on account of a sudden increase in the relative velocity of 

 the current in the upper and lower portions. 



After making a critical examination of the island and the 

 position of the machinery, and also in regard to any obstacle 

 which might interfere with the propagation of the sound, the 

 keeper was directed to put the instrument in operation and 

 to continue to sound it for at least two hours, or until the 

 steamer was lost sight of; which direction was complied with. 

 In passing from the island, almost directly against a light 

 wind, the intensity of the sound gradually diminished as a 

 whole — with the increase of distance, but varied in loudness 

 from blast to blast, now louder, then again more feeble, until 

 it finally ceased at a distance of about fifteen miles, as esti- 

 mated by the intervals between the blasts and the sight of 

 the steam as seen through a spy-glass, and also from points 

 on the Coast-Survey charts. 



