APPENDIX. Y 



liead-quarters of the 2d corps itself, although the latter was 

 about midway between the former and the point of conflict. In- 

 deed the sound appeared so near Gen. Meade's camp that the 

 impression was made that the enemy had gotten between it and 

 Gen. Warren's command. In fact so many instances occurred 

 of wrong impressions as to direction and distance derived from 

 the sound of guns that little reliance came to be placed on these 

 indications. 



In the report of a series of experiments made under the direc- 

 tion of the Light JHouse Board by Gen. Duane of the Engineer 

 Corps is the following remark : " The most perplexing difficulty 

 arises from the fact that the fog-signal often appears to be sur- 

 rounded by a belt varying in radius from one to one and a half 

 miles. Thus in moving directly from a station the sound is au- 

 dible for the distance of a mile, is then lost for about the same 

 distance, after which it is again distinctly beard for a long 

 time." 



Again, in a series of experiments at which Sir Frederic 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 pla ed, the sound entirely dis- 

 appeared, but by going further off to the distance of two or three 

 miles it reappeared in full force, even with a large island inter- 

 vening. Again, from the experiments made under the immedi- 

 ate direction of the present chairman of the Light House Board, 

 Avith the assistance of Admiral Fow^ell and Mr. Lederle, the Light 

 House Engineer, and also from separate experiments made by 

 Gen. Duane, it appears that while a reflector, in the focus of which 

 a steam whistle or ordinary bell is placed, reinforces the sound 

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

 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 iner that the lateral 

 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 issuing through an orifice, although 

 at first proceeding, like a beam of light in parallel rays, soon 

 begins to divergeand spread out into a cone, and at a sufficient 

 distance may include even the entire horizon. 



We may mention also in this connection that from the general 

 fact expressed by the divergence of the rays of sound, the appli- 

 cation of reflection as a means of reinforcing sound must in a 

 considerable degree of necessity be a failure. 



By the application of the principle we have stated and the effect 

 of the wind in connection with the peculiarities of the topography 

 of a region and the position of the sounding body, we think that not 

 4 (49) 



