1875] WRITINGS OF JOSEPH HENRY. 461 



to the ears of the observers against the wind. Starting 

 about 11 :45 a. m. and steaming about two miles, the sound 

 was heard on the tower during twelve minutes and at the 

 foot of the cliff during five and one-half minutes, making a 

 difference of six and a half minutes in favor of audition on 

 the tower. Previous to this experiment the wind had veered 

 one point to the west, bringing the direction of the sound to 

 the observers in less direct opposition to the wind than in 

 the last experiment. 



Sixth experiment. — In this case the steamer was directed 

 to proceed in the opposite direction, or against the wind, so 

 that the sound of the whistle would reach the ear of the ob- 

 servers in the same direction as that of the wind. It started 

 at 12:19 p. m. and proceeded two and one-sixth miles; the 

 whistle was heard during thirteen minutes on the top of the 

 tower, and at the bottom of the cliff during precisely the 

 same time, the difference between the top of the tower and 

 the bottom of the cliff in this case being nothing. 



Seventh experiment. — The vessel having again been called 

 off on other duty the next experiment was made the 1st of 

 September, On this day the wind was north-east ; the veloci- 

 ty at the top of the tower was thirteen and a half miles per 

 hour, and at the bottom of the tower eleven miles per hour. 

 The barometer indicated 30'2 inches pressure, the dry bulb 

 72°, and the wet bulb 67-5°. 



The theoretical conditions for exhibiting the effect of 

 height on audition in this experiment were much more 

 favorable than any of the preceding. First, the velocity of 

 the wind was greater ; second, the difference between the 

 velocities at top and bottom of the tower was well marked, 

 and the direction of the wind was more favorable for direct 

 opposition to the sound as it came to the ear of the observer. 

 In this case, General Woodruff was the observer at the 

 bottom of the cliff, while the chairman of the Light-House 

 Board and his assistant, with several visitors, were at the top 

 of the tower. 



The steamer started at 10 : 58 a. m. and proceeded during 

 eight minutes, or a mile and one-third, when the sound was 



