IV APPENDIX. 



ing, especially in directing the line of research and subsequent 

 application of principles. 



His explanation rests upon the fact that during the passage of 

 a wind between the observer and the sounding instrument the 

 velocity of this will be more retarded at the surface of the earth 

 ■on account of friction and other obstacles, and that the velocity 

 of the stratum immediately above will be retarded by that below, 

 and so on, the obstruction being lessened as we ascend through 

 the strata. From this it follows that the sound wave will be de- 

 formed and the direction of its normal clianged. Suppose, for 

 example, that the wind is blowing directly from the observer. 

 In this case the retardation of the sound wave will be greater 

 above than below, and the upper part of the wave-front will 

 be thrown backwards so that the axis of the phonic ray will 

 be deflected upwards, and over the head of the observer. If, on 

 the other hand, a deep river of wind is blowing directly towards 

 the observer, the upper part of the front of the wave wnll be in- 

 clined down and towards him, concentrating the sound along the 

 surface of the earth. 



The science of acoustics in regard to the phenomena of sound 

 as exhibited in limited spaces has been developed with signal 

 success. The laws of its production, propagation, reflection, and 

 refraction have been determined with much precision, so that 

 we are enabled in most cases to explain, predict, and control the 

 phenomena exhibited under given conditions. But in case of loud 

 sounds and those which are propagated to a great distance, such 

 as are to be employed as fog-signals, considerable obscurity still 

 exists. As an illustration of this I may mention the frequent 

 occurrence of apparently abnormal phenomena. Gen. Warren 

 informs me that at the battle of Seven Pines, in June, 1862, 

 near Richmond, Gen. Johnston, of the Confederate army, was 

 within three miles of the scene of action with a force intended to 

 attack the flank of the Nortbei'n forces, and although listening at- 

 tentively for the sound of the commencement of the engagement, 

 the battle, which was a severe one, and lasting about three 

 hours, ended without his having heard a single gun. (See John- 

 ston's Report.) Another case of a similar kind occurred to Gen. 

 McClellan at the battle of Gaines' Mills, June 21, 1862, also 

 near Richmond. Although a sharp engagement was progressing 

 within three or four miles for four or five hours, the general and 

 his staff were unaware of its occurrenoe, and when their attention 

 was called to some feeble sound they had no idea that it was 

 from anything more than a skirmish of little importance. (See 

 Report of the Commission on the Conduct of the War.) A third 

 and perhaps still more remarkable instance is given in a skirmish 

 between a part of the 2d corps under Gen. Warren and a force 

 of the enemy. In this case the sound of the firing was heard 

 more distinctly at Gen. Meade's head-quarters than it was at the 



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