354 MEMORIAL OF JOSEPH HENRY. 



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 whenever 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 serviceable in giving prominent 

 direction to sounds within moderate or ordinary distances, yet from 

 the rapid diffusibility of the sound-beams, such appliances are 

 worthless for distances beyond a mile or two. 6th. 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. 7th. Intervening obstructions 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 curva- 

 ture 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 favoring wind. 



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, 



