KESEAECHES 'iN SOUND. 517 



duration of the eclio was on tlie average about eight seconds, beginning 

 with the time of its first i)erception, and not with the cessation of the 

 sound of the trumpet. General Woodruff and Doctor Welling both noted 

 the peculiar character of the echo, which was that of a series of reflec- 

 tions varying in intensity from a maximum, near the beginning, and 

 gradually dying away. The wind was nearly at right angles to the axis 

 ot the trumpet and also to that of the crests of the swell of the ocean, 

 which was rolling in from the effects of a commotion without. The 

 barometer at 12 m. indicated 30.2 inches; the dry-bulb thermometer 

 73° F. the wet-bulb 70° F. indicating a remarkable degree of aqueous 

 saturation. During the whole day the air in all the region around 

 Block Island was undoubtedly in a homogeneous condition. 



August G. — On this day the weather was nearly the same. The fog- 

 signal on the oth instant was kept in operation for the use of the mariner 

 nineteen hours, and on this day it was blown twenty hours continuously. 

 The barometer marked 30.20 inches ; the thermometer 70° F. ; the fog 

 not as eciually distributed as on the preceding day ; the north end of the 

 island, distant four miles, being distinctly visible. The wind was S. W. 

 to S., making an angle of about 00° with the axis of the fog-trumpet. 

 The echo continued to be heard distinctly with a sound varying in 

 intensity, but was not as loud as we have heard it on certain occasions 

 in x)re^'ious years. 



During this and the preceding day, workmen were employed under 

 Mr. Brown in inserting a flexible India-rubber tube, two inches in diam- 

 eter, between the revolving plate of the siren and the smaller end of the 

 trumpet, so that it might be brought into a vertical position. This work, 

 though apparently simple, was difficult in execution, since it involved 

 the necessity of strong su^^ports for the cast-iron trumpet, which in itself 

 weighed eight hundred pounds, and also of a union of the parts of suf- 

 ficient strength to resist the pressure of the steam at fifty pounds to the 

 square inch. 



August 7. — Wind from the S. S. W. Fog continued ; the workmen had 

 not as yet completed the attachment. 



August 9. — Barometer 30.30 inches at 12 M. Dry -bulb thermometer 

 74° F. ; wet bulb 71^.5. Wind S. S. W. Fog dense along the south 

 coast, but light over all the northern portion of the island. The echo 

 was heard all day, not very loudly, but distinctly. Siren still horizontal, 

 the arrangement for elevating it not having been, at 10 a. m., completed. 

 Experiments were made on the reciprocal sounds of the whistles from 

 two steamers, the results to be given hereafter. At 5 p. m. the adjust- 

 ment of the flexible tube to the smaller end of the trumpet was finished, 

 which, giving an additional length to the instrument of about 5 feet, 

 threw it out of unison with the sken proper. To restore this unison the 

 t«peed of revolution of the perforated plate was diminished, and after this 

 the trumpet, still being horizontal, was sounded. An echo similar in 



