1874] WRITINGS OF JOSEPH HENRY. 425 



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 ap- 

 paratus to be tested, the plane of the mouth of the trumpet 

 being vertical and the membrane being horizontal; 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 upper end of which is closed by a lens. When 

 the apparatus under examination is sounded, (being suffi- 

 ciently near,) the sand is agitated; the testing instrument 

 is then moved farther off step by step until the agita- 

 tion just ceases; this distance being measured is taken 

 as the relative penetrating power of the sounding appa- 

 ratus. The same process is repeated with another sound- 

 ing 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 apparatus, and so on. 

 On comparing the results given by this instrument with 

 those obtained bj'' 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 penetrating power of two fog-signals may be ob- 

 tained within a distance of a few hunded yards, while to 

 compare them 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. 



