DISCOURSE OF W. B. TAYLOR. 353 



side of the small island, by directing the course of the steamer 

 northward ; and in this case the shore signal was distinctly heard 

 throughout the trip, while the signal from the vessel passed through 

 the " belt of silence " to the observers at the station. The hypothesis 

 of a local sound shadow of definite extent, is excluded by the 

 simple fact that the regions traversed were entirely unobstructed, 

 the two points of observation movable and stationary being 

 constantly in view from each other when not obscured by fog. 

 The hypothesis of a stationary belt of acoustic opacity is equally 

 excluded by the uninterrupted transmission of sound through the 

 critical region in one direction ; and this too whichever order of 

 observation be selected. So that in one of the cases the powerful 

 whistle ten inches in diameter blown by a steam pressure of 60 

 pounds, failed utterly to make itself heard, while the sound from a 

 much feebler Avhistle only six inches in diameter and blown by a 

 steam pressure of 25 pounds, traversed with ease and fulness the 

 very same space. The only hypothesis left therefore is that of 

 diacoustic refraction ; by which the sound-beam from one origin is 

 bent and lifted over the observer, while from an opposite origin the 

 refraction is in a reversed direction; and such a quality in the 

 moving air is referable to no other observed condition but that of 

 its motion, that is to the influence of the wind. Observations were 

 afterward made at Monhegan island, on some of the more normal 

 effects of the refraction of sound by differences of wave velocity, 

 all fully confirming the supposition which had been so variously 

 and critically subjected to examination. 



The principal conclusions summed up in the last Report for 

 1877, are: 1st. The audibility of sound at a distance depends 

 primarily upon the pitch, the intensity, and the quantity of the 

 sound : the most efficient pitch being neither a very high nor a very 

 low one, the intensity or loudness of sound resulting from the 

 amplitude of the vibration, and the quantity of sound resulting 

 from the mass of air simultaneously vibrating. 2nd. The external 

 condition of widest transmission of sound through the air is that 

 of stillness and perfect uniformity of density and temperature 

 throughout. 3rd. The most serious disturbance of the audibility 

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