THE REFKACTION OF SOUND, 207 



We have as tbe laws of sound -motiou in a perfect gas : 



1st. Its velocity is directly proportional to tbe square root of the elas- 

 ticity of the air. 



2(1. Its velocity is inversely proportional to the square root of the den- 

 sity of the air. 



3d. Its velocity is directly proportional to the square root of the abso- 

 lute temperature of the air. 



Tbe theoretical case of luiequal elasticity of the medium, presenting 

 no practical examples, excepting in tbe passage of sound from water 

 into air obliquely, or from air into water, may be here neglected ; and 

 t be remaining actual conditions of acoustic refraction are limited to three, 

 \\z: those of density-inequality, wind-inequality, and teuqierature- 

 iuecpiality. It is important to observe tbat the two fundamental princi- 

 jiles underlying the discussion of acoustic refraction of wbatever origin 

 are, lirst, that the directions of progressive impulse are always at right 

 angles to the surface of tbe wave-front, and, secondly, that any deforma- 

 tion of tbe spherical surface of tbe wave-front must accordingly deflect 

 tbe line of acoustic proi)agation from its original radial direction. 



1. — REFRACTION FROM INEQUALITY OF DENSITY. 



In 1852, Mr. Carl Sondbauss was the first to demonstrate acoustic 

 refraction, and be exhibited it by means of a lens of carbonic-acid gas. 



It may be here preuiised that, in accordance with tlie previous sum- 

 mary, hydrogen, having at tbe })ressure of tbe atmosphere the same elas- 

 ticity, should, from this circumstance alone, transnjit sound with tbe 

 same velocity as ordinary air; but as its rarity is fourteen times greater, 

 tbe velocity of sound in this medium is increased neaily four times, so 

 that wbile at ordinary temperature (05o F.) sound in air would move 

 over 1,125 feet in a second, or one mile in 4:.7 seconds, it would move 

 over 4,250 feet per second (or one mile in 1| second) in an atmosphere 

 of hydrogen. On the other hand, as carbonic ackl, at tbe same pressure 

 and with nearly tbe same elasticity, has a density rather more than 50 

 per cent, greater than air, it would retard tbe velocity of sound about 

 onelittb, or reduce it to 912 feet per second, or one niile in 5.7 seconds. 



]\lr. Sondbauss first employed a thin membranous balloon formed of 

 gold beaters' skin, but obtained more detnded results by forming the 

 env«'lo[)e of a <louble convex iens with two spherical segments of col- 

 lodion film, attached to a leaden hoop, in which were suitable openings 

 lor tlie introduction of the gas. The ticking of a watch placed at some 

 distance behind such lens was heard njost distinctly at a focal point in 

 front of the lens. 



" In ord(T to arrive at a more certain decision, the experiment was 

 arranged in such n)anner tbat whik^- tbe observer sat at the otber side 

 of the lens with closi'd eyes and listened for the ticking of the wat(;h, 

 tbe lens was alternately removed and again brought into position, 

 wbereby it was shown that the ticking of the watch disappeared every 



