496 RESEARCHES IN SOUND. 



air by the opening and sliuttiug of the orifices and consequent rushing out 

 at intervals with explosive violence of the steam or condensed air. The 

 instrument, as originally invented by Cagniard de Latour, of France, 

 was used siniplj- in experiments in physics to determine the pitch of 

 sound ; but Mr. Brown, of I^ew York, after adding a trumpet to it, and 

 modifying the openings in the head of the drum and the revolving plate, 

 oifered it to the Light-House Board 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 diameter at the larger end, and about 1 inch at 

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

 right angles to the axis of the larger end ; on the smaller end is soldered 

 a cone, of which the larger end is about 2 inches in diameter. Across 

 the mouth of this cone is stretched a piece of gold-beater's skin. When 

 the instrument is used, the opening on the larger end is held before 

 the instrmeut to be tested, the membrane being horizontal, and the mouth 

 of the trumpet vertical ; over the membrane is strewed a small quantity 

 of fine sand, which is defended from the agitation of the air by a cylinder 

 of glass, the ui^per end of which is closed by a lens. When the instru- 

 ment under examination is sounded, being sufficiently near the sand, is 

 agitated ; it is then moved further off, step by stej), until the agitation 

 just ceases; this distance, being measured, is taken as the relative pene- 

 trating power of the sounding instrument. The same process is repeatetl 

 with another sounding instrument, and the distance at which the sound 

 ceases to produce an effect on the sand is taken as the measure of the 

 penetrating power of this instrument, and so on. On comi3aring the re- 

 sults given by this instrument with those obtained by the ear on going out 

 a sufficient distance, the two are found to agree precisely in their indica- 

 tions. The great advantage in using this contrivance is that the rela- 

 tive penetrating power of two instruments may be obtained within a 

 distance of a few hundred yards, while to compare the relative power 

 of two fog-signals by the ear requii'es the aid of a steamer and a de- 

 parture from the origin of sound in some cases of 15 or 20 miles. 



PAET III.— I3^ESTIGATI0:N"S DUEING 1873 A^^D 1874.* 



Observations on Sound and Fog-Si&nals, in August, 1873. 



Professor Henry, chairman, and Commander Walker, naval secretary 

 of the Light-House Board, left Portland August 12th, 1873, at 3 o'clock 

 p. M. in the steam-tender Myrtle, Captain Foster, for Whitehead light- 

 station, at which place abnormal phenomena of sound had been observed. 



* From tile Report of the LigLit- House Board^ for 1874. 



