424 WRITINGS OF JOSEPH HENRY. [187* 



For the production of a sound of sufficient power to serve 

 as a fog-signal, bells, gongs, &c., are too feeble except in 

 special cases where the warning required is to be heard only 

 at a small distance. After much experience the Light-House 

 Board has adopted for first-class signals, — instruments actu- 

 ated by steam or hot-air engines, and such only as depend 

 upon the principle of resonance, or the enforcement of sound 

 by a series of recurring echoes in resounding cavities. 



Of these there are three varieties. First, the steam-whistle, of 

 which the part called the bell is a resounding cavity, the sound 

 it emits having no relation to the material of which it is com- 

 posed ; one of the same form and of equal size of wood produc- 

 ing an effect identical with that from one of metal. Another 

 variety is the fog-trumpet, which consists of a trumpet 

 of wood or metal actuated by a reed like that of a clarionet. 

 The third variety is called the siren trumpet, which consists 

 of a hollow drum, into one head of which is inserted a pipe 

 from a steam-boiler, while in the other head a number of 

 holes are pierced, which are alternately opened and shut by 

 a revolving plate having an equal number of holes through 

 it. This drum is placed at the mouth of a large trumpet. 

 The sound is produced by the series of impulses given to the 

 air by the opening and shutting of the orifices and conse- 

 quent rushing out at intervals with explosive violence of the 

 steam or condensed air. The instrument, as originally in- 

 vented by Cagniard de Latour, of France, was used simply 

 in experiments in physics to determine the pitch of sound; 

 but Mr. T. Brown, of New York, after adding a trumpet to it 

 and modifying the openings in the head of the drum and 

 the revolving plate, under the direction of the Light-House 

 Board, perfected it as a fog-signal, and as such it has been 

 found the most powerful ever employed. 



In ascertaining the penetrating power of different fog- 

 signals I have used with entire success an instrument of 

 which the following is a description: A trumpet of ordinary 

 tinned iron of about 3 feet in length, and 9 inches in diam- 

 eter at the larger end, and about 1 inch at the smaller, is 

 gradually bent so that the axis of the smaller part is at 



