RECENT EXPERIMENTS ON FOG SIGNALS. 213 



In a paper published in the " Philosophical Transac- 

 tions" for 1876, Professor Osborne Keynolds refers to 

 these echoes in the following terms: " Without attempt- 

 ing to explain the reverberations and echoes which have 

 been observed, I will merely call attention to the fact that 

 in no case have I heard any attending the reports of the 

 rockets,* although they seem to have been invariable 

 with 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 lam not aware that they have ever been observed in 

 the case of a sound which has no direction of greatest in- 

 tensity." 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 pro- 

 duction of the echoes. 



There is not a feature connected with the aerial echoes 

 which cannot be brought out by experiments in the air of 

 the; laboratory. I have recently made the following experi- 

 ment: A rectangle, x Y (p. 214), 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,"f f 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 front of the 

 South Foreland strikingly imitated. J Turning off the gas, 



* These carried 12 oz. of gunpowder, which has been found by 

 Colonel Fraser to require an iron case to produce an effective ex- 

 plosion. 



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



| ' Lectures on Bound,'" 3d ed., p. 268. 



