508 RESEARCHES IN SOUND 



upward in a line slightly curving toward the horizon, in the direction of 

 the wind at the surface, as far as it could he followed with the eye, in- 

 dicating a wind in the same direction in the several strata through which 

 it passed, but of a greater velocity in the upper strata. 



The vessels now changed places, the Cactus steaming west, the Mis- 

 tletoe 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 as- 

 cended vertically until it attained an elevation of about one thousand 

 feet, when, turning east, it followed the direction of the previous one. 

 The sound in this case from the east was heard three miles, while 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 became less. 



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

 its direction 10 points or about 112^° round through the south, and as 

 indicated by the anemometer at 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 Cactns, 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 they were observed to turn east, following the direc- 

 tion of the wind observed in the earlier observations. The results of 

 the whole series of observations are extremely interesting. In all the 

 experiments the difference in the audibility of the sound in different di- 

 rections was very marked, and indeed it rarely ha|)pens that the sound 

 is equal in two directions, although from the hypothesis adopted this 

 may be possible, since, according to this hypothesis, both the upi>er and 

 lower currents have an influence upon the audibility of sound in certain 

 directions. From the tirst trial, the motion of the air being in the same 

 direction both below and above, but probably 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 direction, 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 second day are also in conformity 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 



