458 WRITINGS OP JOSEPH HENRY. [1875 



Experiments as to the Effect of Elevation on Audibility. — For 

 this investigation the first-order Hght-house at Block Island 

 offered peculiar facilities. It is situated near the edge of a 

 perpendicular bluff 152 feet above the sea. The tower being 

 62 feet above the base — gives a total height (to the focal plane 

 of the lens) of 204 feet, on the level of which the ear of the 

 observer could be placed. 



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 oppo- 

 site directions. The barometer indicated 301 inches of 

 atmospheric pressure; the dry -bulb thermometer indicating 

 74° F., and the wet-bulb 69°. The wind at the time of the 

 experiments was from the west, and of a velocity of seven 

 miles per hour. The vessels started from the point C, Fig. 

 4, opposite the light-house. A, about one mile distant, a posi- 

 tion as near the shore as it was considered safe to venture. 

 The Putnam steamed with the wind, the Mistletoe steamed 

 against the wind, each blowing its whistle eyery 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 station, 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, the 

 following results were found. 



First experiment. — The duration of the sound on the tower 

 when coming against the wind was nine minutes, while at 

 the base of the cliff 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 experiment at half speed, and hence 

 the duration of the sound on the tower should be considered 

 as 4i minutes, and the difference in favor of audition on the 

 tower 4 minutes instead of 8, as given by the first record. 



Second experiment. — The sound of the Mistletoe, coming to 

 the observers 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 hear- 



