294 FRAGMENTS OF SOIENCE 



the guns and pistols. These facts suggest that the echoes 

 are in some way connected with the direction given to the 

 sound. They are caused by the voice, trumpets, and the 

 syren, all of which give direction to the sound; but I am 

 not aware that they have ever been observed in the case 

 of a sound which has no direction of greatest intensity." 

 The reference to the voice, and other references in his 

 paper, cause me to think that, in speaking of echoes, 

 Professor Osborne Reynolds and myself are dealing with 

 different phenomena. Be that as it may, the foregoing 

 observations render it perfectly certain that the condition 

 as to direction here laid down is not necessary to the 

 production of the echoes. 



There is not a feature connected with the aSrial echoes 

 which cannot be brought out by experiments in the air 

 of the laboratory. I have recently made the following 

 experiment: A rectangle, x Y (p. 295), 22 inches by 12, 

 was crossed by twenty-three brass tubes (half the number 

 would suffice, and only eleven are shown in the figure), 

 each having a slit along it from which gas can issue. In 

 this way twenty-three low flat flames were obtained. A 

 sounding reed, a, fixed in a short tube was placed at one 

 end of the rectangle, and a "sensitive flame,*** /, at some 

 distance beyond the other end. When the reed sounded, 

 the flame in front of it was violently agitated, and roared 

 boisterously. Turning on the gas, and lighting it as it 

 issued from the slits, the air above the flames became 

 so heterogeneous that the sensitive flame was instantly 

 stilled, rising from a height of 6 inches to a height of 18 

 inches. Here we had the acoustic opacity of the air in 



^ Fully described in my "Lectures on Sound," 3d edition, p. 227. 



