1875] WRITINGS OF JOSEPH HENRY. 483 



it. The curve of this figure is not traced through points 

 at which the sound was absolutely lost, but at which it was 

 heard feebly and with nearl}'^ equal distinctness. 



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

 conformity with our conception of the hypothesis of Pro- 

 fessor 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 difference in the area of audition 

 as exhibited in the several figures, all drawn to the same scale. 

 If we compare for example the curve of Fig. 10 with the 

 inner curve of Fig. 8, it might at first sight be inferrecl that 

 the smallness of the curve in the former case was due to 

 a mottled condition of the atmosphere, which by absorbing 

 the sound — diminished the sphere of audition; but unfor- 

 tunately for this explanation, it would appear from the 

 observations made by the Cactus within the hour of obtain- 

 ing the data for describing the curve, that the air was then 

 in a remarkably favorable condition for the transmission 

 of sound, since on the south-east course of the steamer (as 

 shown in Fig. 10), the siren was heard ten and a half miles, 

 the ordinary limit of the maximum penetrating power of this 

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

 September, the day on which the large curve, Fig. 8, was de- 

 scribed, the greatest distance at which the sound of the same 

 instrument could be heard was eight and a half miles. 



The only difference in the condition of the air observed 

 during 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 ques- 

 tion as to the cause of the difference 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 difference in the conditions which ap- 

 parently affected the phenomenon on the days in question 



