VIU APPENDIX. 



in the head of the drum and the revolving plate, offered 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 fol- 

 lowing is a description. A trumpet of ordinary tinned iron of 

 about three feet in length, and nine inches in diameter at the 

 larger end and about an 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 two inches in diameter. Across the 

 mouth of this cone is stretched a piece of goldbeater's skin. 

 When the instrument is used, the opening on the larger end is 

 held before the instrument to be tested, the membrane being 

 horizontal, and the mouth of the trumpet vertical ; over the mem- 

 brane is strewed a small quantity of fine sand which is defended 

 from the agitation of the air by a cylinder of glass, the upper end 

 of which is closed by a lens. When the instrument under exam- 

 ination is sounded, the sand being sufficiently near is agitated, it 

 is then moved further off step by step 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 

 repeated 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 comparing the results 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 indications. The great 

 advantage in using this contrivance is that the relative penetrat- 

 ing 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 requires the aid of a steamer and a 

 departure from the origin of sound in some cases of fifteen or 

 twenty miles. 



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