536 EESEAECHES IN SOUND. 



and also simultaneously by each vessel as it diverged from the vertical 

 siren. 



We were enabled, in this way, to produce two curves by a reverse 

 process. These are plotted in Fig-. 8, and exhibit a remarkable degree 

 of similarity. The corresponding parts of the two curves, being in each 

 case .reversed, exhibit the fact that, through the same space in opposite 

 directions, the audibility of the sound was similarly increased with the 

 wind and diminished against it. The effect however of the wind in the 

 experiments of this day was less marked than on any in the whole 

 series, and consequently the two curves of audition more nearly ap- 

 proximate circles. 



We can see in this result no other effect than that which would bo 

 produced from a wind flowing with a uniform but slow velocity at the 

 surface, but having a slightly increased velocity above. Had there been 

 no wind, according to this view the two curves would have exhibited 

 two concentric circles. 



Experiments of September 4.— Barometer, 29.85 inches, falling ; ther- 

 mometer, dry bulb, 77^ F. wet bulb, 73^.25. Wind south by west, 

 twelve and one-fourth miles per hour at the top of the tower and nine 

 and one-fourth miles below ; variable. 



These experiments were also made with two vessels. The distances 

 and dii^ections are given in Fig. 9. With the exception of the fourth 

 course of the Cactus, the other courses would form nearly ^ symmetri- 

 cal curve, but in this case the sound of the whistle of the Cactus was 

 lost at the point a at a distance of one mile, and was afterward regained 

 at the point ft, and continued audible until the steamer reached the 

 point c. 



This presents one of the abnormal phenomena of sound which might 

 in part be accounted for by the existence of a flocculent cloud between 

 a and &, but why tl^e sound could be heard so much farther in this direc- 

 tion than in the others is not easy to explain on that hypothesis. 



The line h c was described after all the lines of Fig. 9 had been com- 

 pleted, and therefore the curve given in the figure correctly represents 

 the boundary of the area of audition while these courses were being run, 

 the point a being the termination under that condition of the fourth 

 course of the Cactus. To explain the abnormal hue h c, we have only to 

 suppose that a change in the velocity of the wind afterward took place, 

 by which its opposition to the sound-wave was diminished ; this will ac- 

 count for the greater length of the hue ; the change however did not 

 reach the light-house until after the vessel had passed the point b. 



