352 BULLETIN OF THE 



primarily upon the pitcli, tlie intensity, and tlic quantity of tlic 

 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 result- 

 ing 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 tem- 

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

 audibility of sound at a distance, results from its refraction by 

 the wind, which as a general rule moving more freely and rapidly 

 above than near the earth, tends by this difference to lift the 

 sound-beams upward when moving against the wind, and in a 

 downward curve when moving with it. 4th. When the upper 

 current of air is adverse to the lower or sensible wind, or when- 

 ever from any cause the wind below has a higher velocity than 

 that above — in the same direction, the reverse phenomenon is 

 observed of sound being heard to greater distances in opposition 

 to the sensible wind than it is when in the direction of the surface 

 wind. 5th. While suitable reflectors and trumpet cones are ser- 

 viceable in giving prominent direction to sounds within moderate 

 or ordinary distances, yet from the rapid diffnsibility of the sound- 

 beams, such appliances are worthless for distances beyond a mile 

 or two. Gth. The siren has been frequently found to have its 

 clearest penetration through a widely extended fog, and also 

 through a thick snow-storm of large area. Vth. Intervening ob- 

 structions produce sound shadows of greater or less extent, which 

 however at a distance but slightly enfeeble the sound owing to 

 the lateral diffusion and closing in of the sound waves. 8th. The 

 singular phenomenon of distinct audibility of sound to a distance 

 with a limited intermediate region of inaudibility where no optical 

 obstruction exists, is due sometimes to a diffusion of upper sound- 

 beams which have not suffered the upward refraction; sometimes 

 to the lateral refraction of sound-beams or to the lateral spread 

 of sound from directions not affected by the upward refraction; 

 and very frequently to a double curvature of the refracted sound- 

 beams under an adverse lower wind, by reason of the wave fronts 

 being less retarded by the lower or surface stratum of wind than 

 by that a short distance above, and at still greater heights being 

 again less retarded, and finally accelerated by the superior favor- 

 ing wind. 



'O 



These remarkable series of acoustic investigations undertaken 

 after the observer had considerably exceeded his three score years, 



perseveringly continued weeks at a time, and sometimes for 



more than a month, — extending through a period of twelve years, 

 and pursued over a wide and extremely irregular range of sea- 

 coast, and under great variety of both topographical and meteor- 

 ological conditions — untiringly prosecuted by numberless sea 



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