258 



Mr. G. Bischof on 



[Mar. 28, 



There is not a feature connected with the aerial echoes which 

 cannot be brought out by experiments in the laboratory. I have 

 recently made the following experiment : — A rectangle, 22 inches 

 by 12, is crossed by 23 brass tubes, each having a slit along it 

 from which gas can issue. In this way, 23 low, flat flames are ob- 

 tained. A sounding reed, fixed in a short tube, is placed at one end 

 of the rectangle, and a sensitive flame at some distance beyond the 

 other end. When the reed sounds, the flame in front of it is violently 

 agitated, and roars boisterously. Turning on the gas, and lighting it 

 as it issues from the slits, the air above the flames becomes so hetero- 

 geneous that the sensitive flame is instantly stilled by the aerial re- 

 flection, rising from a height of 6 inches to a height of 18 inches. 

 Here we have the acoustic opacity of the' air in front of the South 

 Foreland strikingly imitated. Turning off the gas, and removing the 

 sensitive flame to some distance behind the reed, it burns there tran- 

 quilly, though the reed may be sounding. Again lighting the gas as 

 it issues from the brass tubes, the sound reflected from the hetero- 

 geneous air throws the sensitive flame into violent agitation. Here we 

 have imitated the aerial echoes heard when standing behind the syren- 

 trumpets at the South Foreland. The experiment is extremely simple, 

 and in the highest degree impressive. 



March 28, 1878. 



Sir JOSEPH HOOKER, K.C.S.I., President, in the Chair. 



The Presents received were laid on the table, and thanks ordered 

 for them. 



Notice was given that the name of Dr. Radcliff e had been with- 

 drawn from the list of Candidates for the Fellowship. 



The following Papers were read : — 



I. " On Putrescent Organic Matter in Potable Water. II." By 

 Gustav BiSCHOF. Received February 18, 1878. Commu- 

 nicated by E. Frankland, F.R.S. 



Referring to the paper which I communicated to the Royal Society 

 last session*, I have to add, that after passing water continuously for 

 six weeks through one of the vessels there described, filled with spongy 



* " Proceedings," vol. sxvi, p. 152. 



