538 



EESEAECHES IN SOUND. 



by a cloud passing the zenith. During the second course the wind was 

 variable, changing- its direction about OO'^, principally from the north- 

 west ; while during tlie third course the wind was again from the south- 

 west. The long course of the Cactus marked on the figure indicates the 

 sound of the siren, from the center outward, as it was heard seven and 

 one-fourth miles, then lost for an interval, and afterward heard again at 

 a distance of three and one-fourth miles farther, making, in all, ten and 

 one-half miles. 



Fific. 10. 



\ -».. 



^^\ 





Uxperiments of September 7. — Barometer, 30.1 inches ; thermometer, 

 dry-bulb, 73° F. wet-bulb, 62° ; wind, eight miles per hour above, and 

 five miles per hour below, tower. The wind was variable, as indicated 

 by the letting-oif of balloons, which however did not rise to any great 

 height. The direction of the wind is shown 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 ; thermometer, 

 dry -bulb, 71° F. wet-bulb, 64^.5; wind, west-southwest, fifteen miles 

 per hour at top of tower, nine miles per honr below. Fig. 12 indicates 

 the curve of audition of the vertical siren as compared with that of the 



