462 WRITINGS OP JOSEPH HENRY, [1875 



lost at the top of the tower. In this case, though the sound 

 was heard for eight minutes at the top of the tower, and 

 the first five blasts marked on the notes as quite loud, it was 

 not heard at all at the bottom of the cliff, at least a hundred 

 yards nearer the source of the sound. 



This result, which interested and surprised a number of 

 intelligent visitors, who were in the tower at the time, strik- 

 ingly illustrates the effect of elevation on the audibility of 

 sound moving against the wind. The result was so impor- 

 tant that it was thought advisable to immediately repeat the 

 experiment under the same conditions. 



Eighth experiment. — The Mistletoe was again directed to 

 proceed, in the direction of the wind, along the line it had 

 previously traversed. It started at 11:25 a. m., and pro- 

 ceeded during six minutes, or one mile, when the sound 

 was lost at the top of the tower. In this case, the first blast 

 of the whistle was feebly heard at the base of the cliff", but 

 no other, while thirteen blasts were heard at the top of the 

 tower, of which the first six were marked as loud. 



That this remarkable effect was not produced by an 

 acoustic cloud or a flocculent atmosphere is evident from the 

 experiment which immediately succeeded. 



Ninth experiment. — In this trial, the Mistletoe was directed 

 to proceed against the wind, so that the sound of its whistle 

 should come to the ears of the observers with the wind. 

 It started at 11:48 a. m., and proceeded during sixteen 

 minutes, or two and two-thirds miles, when the sound of its 

 whistle was lost to the observers on the top of the tower. 

 In this case the sound of the whistle became audible at the 

 bottom of the cliff" as soon as the position of the vessel became 

 such as to bring the sound to the observers approximately 

 with the wind, and continued to be audible during fifteen 

 minutes, or within one minute as long as the sound was 

 heard at the top of the tower. 



It may be mentioned as an interesting fact that an assis- 

 tant who was observing the sound with General Woodruff" 

 at the foot of the cliff", when the sound could not be heard at 

 the level of the sea (in the sixth experiment), perceived it 



