428 WRITINGS OF JOSEPH HENRY. [1874 



The result of this investigation clearly showed the power 

 of the apparatus in propagating sound under conditions not 

 entirely favorable, since the wind — though light, was in 

 opposition to the sound. 



Cape Elizabeth Light-Station, Maine, August 29, 1873. — The 

 fog-signal at this place is on a prominent headland to 

 which the course of all vessels is directed when bound 

 from the southward into Portland Harbor. It is furnished 

 with two light-houses 919 feet apart and 143 feet above sea- 

 level. The easterly tower is connected with the keeper's 

 dwelling by a wooden-covered way 200 feet long and about 

 12 feet high ; the station is furnished with a 10-inch steam 

 fog-whistle, placed to the southward of the easterly tower at 

 a distance of about 625 feet and about at right angles with 

 the covered way; it therefore has a background, including 

 the covered way, of about 65 feet above the height of the 

 whistle, which was found to reflect a perceptible echo. The 

 whistle was actuated by steam at 55 pounds pressure, con- 

 suming from 60 to 65 pounds of anthracite coal per hour. 

 The whistle itself differs from the ordinary locomotive- 

 whistle by having a projecting ledge or rim around the 

 lower part through which the sheet of steam issues to strike 

 against the lower edge of the bell. What effect this project- 

 ing ledge or rim may have is not known to the observers. 

 This whistle is provided (for the purpose of concentrating 

 the sound in a given direction) with a hollow truncated 

 pyramid 20 feet long, 10 feet square at the large end, and 2|- 

 feet square at the small end, the axis of the pyramid being 

 placed parallel to the horizon, with the whistle at the smaller 

 end. In order to ascertain the effect of this appendage to 

 the whistle, the simplest plan would have been to note the 

 intensity of sound at various points on a circle of which 

 the whistle should be the centre. This being impractica- 

 ble on account of the intervention of the land, the ob- 

 servations were confined to points on the three arcs of a 

 circle of about 120°, of which the axis divided the space 

 into 80° and 40°, and a radius of one, two, and three miles. 

 The result of these observations was that starting from the 



