442 WRITINGS OF JOSEPH HENRY, [1874 



wind at the surface, as far is it could be followed with the 

 eye, indicating a wind in the same direction in the several 

 strata through which it passed, but of a greater velocity in 

 the upper strata. 



Second trial. — The vessels now changed places, the Cactus 

 steaming west, the Mistletoe east, the wind having entirely 

 ceased at the surface of the earth. In this case the Cactus 

 lost the sound of the Mistletoe when the vessels were two 

 miles apart, while the Mistletoe continued to hear the sound of 

 the Cactus until they were three miles apart. A balloon let 

 off ascended vertically until it attained an elevation of about 

 one thousand feet, when turning east it followed the direc- 

 tion of the previous one. The sound in this case from the 

 east was heard three miles, and that from the west was 

 heard two miles, while in the preceding observations the 

 distances were as 2 to 1 ; the only changing element (as 

 far as could be observed) was that of the wind at the surface, 

 which had somewhat diminished. 



Third trial 12:45 p. m. — The wind previous to this trial 

 had changed its direction 10 points or about 112J° round 

 through the south, and (as indicated by the anemometer) had 

 a velocity of 4*8 miles per hour. In this case the Cactus, 

 going against the wind, lost the Mistletoe^s sound coming to 

 her against the wind, when the vessels were 1 mile apart, 

 while the Mistletoe heard the Cactus, the sound coming to 

 her with the wind, when the vessels were 1| miles apart. 

 The several balloons set off at this time were carried by the 

 surface wind westwardly until nearly lost to sight, when 

 the}'' were observed to turn east, following the direction of the 

 wind that prevailed below in the earlier observations. The 

 results of the whole series of observations are extremely in- 

 teresting. In all the experiments the difference in the audi- 

 bility of the sound in different directions was very marked, 

 and indeed it rarely happens that the sound is equal in two 

 directions, although from the hypothesis adopted, this may 

 be possible, since according to it both the upper and lower 

 currents have an influence upon the audibility of sound in 

 certain directions. 



