350 BULLETIN OP THE 



effect of altitude in modifying unfavorable conditions of audibility. 

 Observers were accordingly stationed on tlie beach at the foot of 

 the cliff, and also on the tower 200 feet above, to record simul- 

 taneously the duration of the whistle signals of two steamers pro- 

 ceeding in opposite directions toward tiie right and the left. The 

 sound coming against the wind (of about seven miles per hour) 

 continued audible at the upper station four times longer, (^. e., 

 for four times greater distance) than at the lower station. The 

 sound coming with the wind, was unexpectedly heard at the lower 

 station for a longer period than at the upper one. Another ob- 

 servation (with the wind about five miles per hour) gave for the 

 sound against the wind, rather more than twice the distance of 

 audibility at the upper station; and for the sound favored by the 

 wind, a slightly greater distance at the top than at the bottom 

 station. The next observation gave as before, with the adverse 

 wind, the advantage of more than double the distance of audibility 

 to tiie upper station ; meanwhile one of the observers at the foot 

 of the cliff, after the sound was entirely lost, managed by climbing 

 to a ledge about 30 feet above the beacli, to recover tiie signal 

 quite distinctly, and to hear it for some time. The sound coming 

 with the wind continued to be heard at both the higher and the 

 lower stations for precisely the same time, giving on this occasion 

 no advantage to either. Observations made on board the two 

 steamers while moving in opposite directions, gave for the sound 

 travelling with tiie wind a duration and distance more than five 

 times that for the sound which came against the wind. Five 

 similar experiments gave very similar results. The two vessels 

 moving in opposite courses, each at right angles to the direction 

 of the wind, gave a very close equality for the reciprocal dura- 

 tions of the sound. In the following month, similar observations 

 were made at Little Gnll Island, which were very accordant with 

 those made at the former station. As a result of plotting the 

 ranges of audibility in different directions from a given point, 

 producing a series of circular figures (more or less distorted) of 

 very different sizes, Henry was inclined to believe that the whole 

 urea of audition is less in high winds than in gentle winds. These 

 investigations as their author well remarks, — "though simple in 

 their conception have been difficult and laborious in their execu- 

 tion. To be of the greatest practical value they were required to 

 be made on the ocean under the conditions in which the results 

 are to be applied to tlie use of the mariner, and therefore they 

 could only be conducted by means of steam vessels of sufficient 

 jiower to withstand the force of rough seas, and at times when 

 these vessels could be spared from other duty. They also required 

 a number of intelligent assistants skilled in observation and faith- 

 ful in recording results."* 



* Report of the Light-house Board U. S. for 1875, p. 107. 



124 



