W. B. Taylor — Recent Researches in Sound. 33 



the island from the steamer, with a large house and rising 

 ground interposed, Professor Henry infers that the region of 

 inaudibility was covered by an acoustic shadow, encroached 

 upon at a' greater distance by the divergence of the rays of 

 sound, which, bending, reached ultimately the surface of the 

 water, (p. 107.) A similar phenomenon was observed in the 

 same year on approaching Whitehead station near the coast of 

 Maine. The fog-signal was heard from the distance of six 

 miles to about three miles, and then lost until within a quarter 

 of a mile. (p. 107.) Again, at little Gull Island, in a vessel re- 

 ceding from the siren signal in the direction of its trumpet 

 axis, the sound was lost at a distance of two miles, and then 

 regained at a distance of four and a half miles, (p. 111.) These 

 last cases are referred by Professor Henry to a flexure of the 

 rays of sound resulting from differences of wind velocity in the 

 upper and lower strata of air. 



[9.] In 1872, it was observed that a fog-signal was heard from 

 one station to another, while a simultaneous signal from the 

 latter was inaudible in the opposite direction. On board a 

 steamer approaching Whitehead station (a mile and a half from 

 the coast of Maine), the signal, a steam-whistle, failed to be 

 heard from the distance of about three miles to about a quar- 

 ter of a mile from the station ; while a smaller whistle on the 

 steamer was distinctly heard by the keeper at the station dur- 

 ing that time. The "wind was slightly transverse to the direc- 

 tion from the steamer to the station, but approximately in that 

 direction. The steamer after ^topping at the station, on passing 

 from it almost directly against a light wind, continued to hear 

 the signal with variable distinctness for about fifteen miles, (p. 

 108.) In September, 1874, the keeper at Block Island, on the 

 coast of Rhode Island, observed according 



times when the fog signal from Point Judith at a distance of 

 seventeen miles was audible, and in comparing the times when 

 tlie Block Island signal (a powerful steam siren) was heard at 

 Point Judith, it appeared that the two sounds had not been 

 heard simultaneously by the two keepers, (p. 112.) 



[10.] In August, 1873, at Cape Elizabeth station in Maine, the 

 phenomenon of ocean-echoes was distinctly noticed on board a 

 steamer as it was passing directly outward from the signal ; the 

 sound after each whistle being returned from the unobstructed 

 space beyond, (p. 109.) In September, 1874, at Black Rock 

 Island also, shortly after each blast of the trumpet, a prolonged 

 eciio from the open ocean was distinctly heard. The echo was 

 observed not to be loudest at the siren -ho use, but at a point sev- 

 eral hundred yards to one side ; the wind being in the direction 

 oi ilie |)rimitive sound, and nearly opposite to the direction of 

 liected echo. (p. 112.) This was supposed by Professor 

 ' >tK. Sci.— Third Series, Vol. XI, No. 61.— Jan., 1876. 



