EESEAECHES IN SOUND. 519 



sound of a feeble character, while the phenomena in nature ai-c produced 

 with a comparatively small difference of temperature and with powerful 

 sounds. 



Experiments at Block Island 



relative to the effect of elevation on audibility. 



For this investigation the first-order light-house at Block Island offered 

 peculiar facilities. It is situated near the edge of a perpendicular bluffy 

 153 feet above the sea. The tower being 52 feet above the base, gives a 

 total height to the focal i^lane of the lens of 204 feet, on the level of which 

 the ear of the observer could be jjlaced. 



The first and second experiments of this class were made on the 10th 

 of August, with two light-house steamers, the Putnam and the Mistletoe, 

 moving simultaneously in opposite directions. The barometer indicated 

 30.10 inches of atmospheric pressure ; the dry-bulb thermometer indi- 

 cating 74P F., and the wet-bulb 09°. The wind at the time of the ex- 

 joeriments was from the west, and of a veloeitj" of seven miles per hour. 

 The vessels started from the point C, Fig. 4, opposite the light-house, 

 A, about one mile distant, a position as near the shore as it was con- 

 sidered safe to venture. The Putnam steamed with the wind, the Mis- 

 tletoe steamed against the wind, each blowing its whistle every half 

 minute. The duration of the sound was noted at the top of the tower and 

 at the level of the sea, Mr. Brown being the observer at the latter sta- 

 tion, while the chairman of the board, with an assistant, observed at the 

 former. On comparing notes, the watches having been previously set 

 to the same time, it was found — 



First. That the duration of the sound on the tower, when coming 

 against the wind, was nine minutes, while at the base of the cliii" it was 

 heard only one minute. It was afterward found from the records on 

 board of the Putnam, the sound of which came against the wind, that 

 this vessel was moving, during the exi)eriment, at half-speed, and hence 

 the duration of the sound on the tower should be considered as 4|^ minutes, 

 and the difference in favor of audition on the tower 4 minutes instead 

 of 8, as given by the first record. 



Second. That the sound of the Mistletoe, coming to the observer with 

 the wind, was heard on the tower during 15 minutes, while it was heard 

 at the base of the cliff during 34 minutes, the difference being 19 minutes 

 in favor of hearing at the level of the sea. This result, which differs from 

 that of all the other experiments of the same class, deserves special at- 

 tention. 



After making the foregoing experiments of this class, and others, on 

 the effect of wind on sound, to be described in the next section, the ves- 

 sels were called off" for other duty, and the investigations were not re- 

 sumed until August 17, when the following experiments were made : 



The third experiment. — The wind was from the E. X. E., at the rate of 



