540 EESEARCHES IN SOUND. 



sliadow of the liglit-liouse tower and the keeper's dwelling. This shadow 

 coutinued to the point e, which was nearly the extent of the acoustic as 

 well as of the optical shadow, since from d to e the sound was heard 

 from neither instrument, and the origin of sound was too near to cause 

 much diflereuce between these two shadows. From / to a, through the 

 point//, the two instruments coutinued to be fully heard — the vertical 

 the more distinct. The efiect of the wind in this figure is also very 

 distinctly marked, the longer lines indicating the distance the sound 

 was heard with the wiud, and the shorter against it. The curve of this 

 figure is not traced through points at which the sound was absolutely 

 lost, but at wJiich it was heard feebly and with nearly equal distinctness. 



Thus far all the facts we have observed, if not in strict conformity 

 with our conception of the hypothesis of Professor Stokes, are at least 

 not incompatible with it. We are now however to direct attention to 

 a fact of much interest, which may not have escaped the attention of 

 the reader; namely, the remarkable difterence in the area of audition 

 as exhibited in the several figm^es, all drawn to the same scale. Com- 

 pare, for example, the curve of Fig. 10 with the inner curve of Fig. 8. 

 It might at first sight be inferred that the smalluess of the curve in 

 the former case was due to a mottled condition of the atmosphere, 

 which, b}' absorbing the sound, diminishes the sphere of audition ; but, 

 unfortunately for this explanation, it would apjiear from the observa- 

 tions made by the Cactus within the hour of obtaining the data for 

 describing the curve, that the air was then in a remarkably' favorable 

 condition for the transmission of sound, since it was heard ten and a 

 half miles, the ordinary limit of the maximum penetrating power of the 

 instrument — a siren of the second order; while on the 3d of September, 

 the day on which the large curve, Fig. 8, was described, the greatest 

 distance at which the sound of the same instrument could be heard was 

 eight and a quarter miles. 



The only difierence in the condition of the air observed dirring the 

 time of describing the curve of audition given in the figure, and the 

 hearing of the sound by the Cactus for ten and a half miles, was a 

 change in the direction, and perhaps in the intensity, of the wind, in the 

 latter case the direction being the same as that of the course of the 

 Cactus. 



Before therefore admitting any other solution of the question as to 

 the cause of the difterence in the area of audition, we must inquire 

 whether it is not possible to refer it to the action of the wind itself. 



The most marked difterence in the conditions which apparently af- 

 fected the phenomenon on the days in question was that of the greater 

 velocity of the wind, both at the surface Of the sea and at the top of the 

 tower, and by comparing the several figures in regard to the wind it 

 will be seen that where the condition of the air was nearest that of a 

 calm the larger was the curve of audition, and the nearer the figures 

 approach to a circle, of which the point of origin of sound or the point 



