482 



WRITINGS OF JOSEPH HENRY. 



[1875 



per hour at top of tower, and five miles per hour below. The 

 wind was variable, as indicated by the letting-off of balloons, 

 which however did not rise to any great height. The direc- 

 tion of the wind is show^n in Fig. 11 by arrows. There is 

 nothing remarkable in the curve of audition of this day. It 

 indicates as usual a greater distance toward the side on which 

 the sound was moving with the wind. 



Experiments of September 8. — Barometer, 30'3 inches; ther- 

 mometer, dry bulb, 70° F., wet bulb, 64"5° : wind, west-south- 

 west, fifteen miles per hour at top of tower, nine miles per 



hour below. Fig. 12 in- 

 dicates the curve of audi- 

 tion of the vertical siren as 

 compared with that of the 

 horizontal siren. The 

 steamer first proceeded 

 along the line C a nearl}'' 

 in the direction of the axis 

 of the horizontal trumpet. 

 For the distance of the first 

 three miles the horizontal 

 trumpet was the louder. 

 At the point a, four miles 

 distant, the two were distinct and very nearly equal. At b 

 they were distinct, also very nearly equal, the vertical per- 

 haps a little more distinct. At c very nearly equally distinct. 

 At d the vertical siren was decidedly more distinct just be- 

 fore entering the optical shadow of the light-house tower 

 and the keeper's dwelling. This shadow continued 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 difference between these two 

 shadows. From / to a, through the point g, the two instru- 

 ments continued to be fully heard — the vertical the more 

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

 distinctly marked, the longer lines indicating the distance 

 the sound was heard with the wind, and the shorter against 



Fig 



