1874] WRITINGS OF JOSEPH HENRY. 421 



the Engineer Corps, is the following remark : " The most per- 

 plexing difficulty arises from the fact that the fog-signal 

 often appears to be surrounded by a belt varying in radius 

 from one to one and a half miles, from which the sound 

 appears to be entirely absent. Thus in moving directly 

 Irom a station the sound is audible for the distance of a mile, 

 is then lost for about the same distance, after which it is 

 again distinctly heard for a long time." 



Again, in a series of experiments at which Sir Frederick 

 Arrow and Captain Webb, of the Trinity Board, assisted, it 

 was found that in passing in the rear of the opposite side of 

 an island in front of which a fog-signal was placed the sound 

 entirel}^ disappeared, but by going farther off to the distance 

 of two or three miles, it re-appeared in full force, even with 

 a large island intervening. Again, from the experiments 

 made under the immediate direction of the present chair- 

 man of the Light-House Board, with the assistance of 

 Admiral Powell and Mr. Lederle, the light-house engineer, 

 and also from separate experiments made by General Duane, 

 it appears that while a reflector in the focus of which a steam 

 whistle or ordinary bell is placed — re-enforces the sound for 

 a short distance, it produces little or no effect at the dis- 

 tance of two or three miles, and indeed the instrument can 

 be as well heard in still air at the distance of four or five 

 miles in the line of the axis of the reflector whether the ear 

 be placed before or behind it. From these results we would 

 infer that the divergency of sound, or its tendency to spread 

 laterally as it passes from its source, is much greater than 

 has been supposed from experiments on a small scale. The 

 idea we wish to convey by this is that a beam of sound issu- 

 ing through an orifice, although at first proceeding like a 

 beam of light in parallel rays, soon begins to diverge and 

 spread out into a cone, and at a sufficient distance may in- 

 clude even the entire horizon. 



We may mention also in this connection, that from the 

 general fact of the divergence of the rays of sound, the ap- 

 plication of reflection as a means of re-enforcing sound must 

 of necessity be in a considerable degree a failure. 



