460 WRITINGS OF JOSEPH HENHY, [1875 



After making the foregoing experiments of this class, and 

 others on the effect of wind on sound, to be described in the 

 next section, the vessels were called off for other duty, and 

 the investigations were not resumed until August 17, when 

 the following experiments were made : 



Third experiment. — The wind was from the E. N. E., at 

 the rate of about five miles per hour at the surface, and a 

 greater velocity at the height of the tower. Barometer, 30" 25 

 ins. ; thermometer, 72°. 



In this and the subsequent experiments of the same day, 

 but one steamer — the Mistletoe — was employed. It started 

 at 10:30 a. m. from the point C, Fig. 4, at the foot of the 

 cliff, and steamed W. S. W. along C B for about 12 minutes, 

 or a distance of two miles, blowing the whistle every half- 

 minute. To note the duration of the sound. Dr. Welling 

 was stationed at the foot of the cliff, at the level of the sea, 

 while the chairman of the Light-House Board, with an assis- 

 tant who acted as clerk, was on the upper gallery of the 

 tower, the ears of the latter being almost precisely 200 feet 

 above those of the observer at the foot of the cliff. 



The watches having been previously set to the same time, 

 on comparing results it was found that the whistle was heard 

 at the top of the tower for twelve minutes and at the bottom 

 of the cliff for five and one-half minutes, making the differ- 

 ence in favor of audition on the tower six and one-half min- 

 utes. In this experiment the sound came to the observers 

 nearly against the wind. 



Fourth experiment. — This consisted in directing the 31istle- 

 toe to proceed in the opposite direction from the same point, 

 along the line C D. It started at 11 : 5 a. m. the breeze being 

 light at the time, and proceeded about two and one-half 

 miles before the sound was lost to the observers. On com- 

 paring notes it was found that the sound was heard at the 

 top of the tower during fifteen minutes, and at the level of 

 the sea for eleven minutes, giving a difference in favor of 

 the hearing on the top of the tower of four minutes. 



Fifth experiment. — In this, the Mistletoe steamed again in the 

 direction with the wind, the sound from its whistle coming 



