1874] WRITINGS OF JOSEPH HENRY. 433 



still continuing to hear the signal distinctly until the keeper, 

 losing sight of the vessel, stopped sounding the instrument. 

 These observations were made under very favorable circum- 

 stances, it being nearly calm. What wind did exist was 

 about equally favorable to points on either side of the axis. 

 The inference from these observations is — first, that small 

 objects placed near the source of sound tend to diminish its 

 intensity in the direction of its interruption, and should 

 therefore if possible be removed, or the instrument so placed 

 as to obviate such obstructions ; and second, that even with 

 the trumpet, the sound so diverges from the axis as to be 

 efficient even in the rear of the instrument. 



Observations on Fog-Signals, in August and September, 1874. 



The first of these investigations was made August 25, on 

 l)oard the steamer Putnam, at Little Gull Island, with Com- 

 modore Stephen D. Trenchard, inspector of lights of the 

 third district, accompanied by Governor Charles R. Inger- 

 soll, of Connecticut, and Captain John H. Upshur, U. S. N. 



At this place are two sirens, the one to replace the other 

 in case of an accident. One of the sirens was sounded with 

 the pressure of 50 pounds per square inch. The wind was 

 across the axis of the trumpet, and almost precisely at right 

 angles to it. 



The steamer was headed against the wind, on a line at 

 right angles to the axis of the trumpet. The sound in this 

 case also travelled against the wind, which was at an esti- 

 mated velocity of from 4 to 5 miles per hour. The distance 

 travelled before the sound became inaudible, was estimated 

 by the speed of the steamer, at 3| miles. 



The steamer was next headed in an opposite direction 

 and returned along its previous path, across the mouth of 

 the trumpet of the siren, the sound gradually increasing in 

 strength without any marked irregularity, until the siren 

 was reached, and on leaving this, (the course remaining the 

 same,) the sound gradually diminished in intensity, but with 

 less rapidity than before, until it was finally lost at a dis- 

 tance of 7^ miles. In the latter instance, the movement of 

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