1874] WRITINGS OF JOSEPH HENRY. 431 



eastward of the observers; then in one instance the sound 

 became very indistinct, and in another was entirely lost, 

 both at a distance of about two miles. In these cases the 

 line of sight between the observers and the signal was inter- 

 rupted — in the first by a small building the gable-end of 

 which was within 10 feet of the whistle, and in the second 

 by the south light-tower, which is within 30 feet of the 

 whistle. In this series of experiments, as in the last, the 

 wind was against the sound; the efifect was noted by passing 

 over the arc several times at different distances. The wind 

 was from the southward and westward and very light, and 

 the sound was finally lost at about six miles, and in the 

 direction of the obstructions. 



Boston Light-Station, August 31, 1873. — The light-house is 

 situated on a low rocky island, on the north side of the main 

 outer entrance to Boston Harbor, nine miles from the city. 

 It is furnished with three caloric engines, two of the second 

 class and one of the first. The two second-class engines are 

 so arranged as to act separately or together, and in the latter 

 arrangement serve to duplicate the larger engine. At the 

 time the observations were made, the larger engine was 

 about being repaired, and one of the smaller engines with 

 the double air-reservoir was used. The larger engine is 

 used with 12 pounds pressure of air, which falls to 8 pounds 

 in producing the sound. The smaller engine, with the 

 double reservoir, is started with 9 pounds pressure, which 

 falls to 8 pounds. This difference in the pressure of air in 

 the two engines is caused by the larger ratio of the reservoir 

 to the size of the reed. With a greater pressure than 12 

 pounds to the square inch in the larger eagine, and 9 pounds 

 in the smaller, no sound is produced ; the reed is unable to 

 act against the pressure, and consequently the orifice remains 

 closed. The trumpet of the larger of the engines is reported 

 to have been heard eighteen miles at sea, which — in consid- 

 eration of the results obtained at Whitehead, we thought 

 very probable. The time required (from starting fires) to 

 get a good working-pressure, is about half an hour. The 

 amount of coal consumed per hour is 17 pounds. 



