DISCOURSE OF W. B. I'AYI.oK. 35] 



the changing surface wind was apparently a land ami sea breeze 

 "due to the heating of the land as the day advanced :" and the 

 varying behavior of the sound-beams was easily explained by the 

 varying differences of velocity in their wave fronts at different 

 heights. 



In 1875 Henry continued his observations at Block island, (R. I.J 

 and at Little Gull island: (Conn.) The southern light-house on 

 Block island standing on the edge of a perpendicular cliff 152 feet 

 above the sea level, and being itself 52 feet high (to its focal plane) 

 this point was selected for making investigations on the effect of 

 altitude in modifying unfavorable conditions of audibility. < (bser- 

 vers were accordingly stationed on the beach at the tout of the cliff, 

 and also mi the tower 200 feet above, to record simultaneously the 

 duration of the whistle signals of two steamers proceeding in oppo- 

 site directions toward the 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, ( i. 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 observation (with the wind 

 about five miles per hour) gave lor the sound against the wind, 

 rather more than twice the distance of audibility at the upper 

 station; and for the sound favored bv the wind, a slightly greater 

 distance at the top than at the bottom station. The next observa- 

 tion gave as before, with the adverse wind, the advantage of more 

 than double the distance of audibility to the upper station; mean- 

 while 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 beach, to recover the signal quite distinctly, and to hear 

 it for some time. The sound coining 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. < >bser- 

 vations made on board the two steamers while moving in opposite 

 directions, gave for the sound travelling with the 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 



