1874] WRITINGS OF JOSEPH HENRY. 443 



In the first trial, (of September 23,) the motion of the air 

 being in the same direction both below and above, but prob- 

 ably more rapid above than below on account of resistance, 

 the upper part of the sound-wave would move more rapidly 

 than the lower, and the wave would be deflected downward, 

 and therefore the sound (as usual) heard farther with the 

 wind than against it. In the third experiment of the same 

 day, in which the wind changed to an almost opposite direc- 

 tion, if the wind remained the same above, — as we have reason 

 to suppose it did from the observations on the balloons on 

 the second day, the sound should be heard still farther in 

 the same direction — or against the wind at the surface, since 

 in this case the sound-wave being more retarded near the 

 surface would be tipped over more above and the sound thus 

 be thrown down. 



The observations of the next day (Sept. 24) are also in con- 

 formity with the same hypothesis, the change in the wind 

 being probably due to the heating of the land — as the day 

 advanced, beyond the temperature of the water, and thus 

 producing a current from the latter to the former, while 

 the wind observed in the morning from the west was the 

 land-wind due to the cooling of the latter. 



In the morning the wind was blowing from the west both 

 in the higher strata and at the surface of the earth, and in 

 this condition the sound was heard farther with the wind 

 than against it. 



The wind at the surface about mid-day gradually ceased, 

 and shortly afterward sprang up from an easterly direction ; 

 in this condition the sound, (with the wind at the surface) was 

 heard at a greater distance. This is also in strict conformity 

 with the theory of a change in the form of the sound-wave, 

 as in the latter case, the lower portion would be retarded, 

 while the upper portion of the wave would be carried for- 

 ward with the same velocity, and hence the sound would be 

 thrown down on the ear of the observer. To explain the 

 result of the third trial of the second day, we have only to 

 suppose that the influence of the upper current was less than 

 that of the lower. The conditions for these observations 



