340 PROPAGATION OF SOUND. 



is held horizontally in a line with the eye, so that the 

 are seen as they gradually recede, or by placing onese 

 opposite to the apparatus and at some distance, so as t 

 see them as they gradually approach. It will be * 

 once seen that the air, although in this case propell 

 with considerable velocity, travels much more slow] 

 than sound. At a distance of from 2 m to 4 m from tl 

 apparatus and opposite to it, an observer does not he r 

 the sound of the knocking sensibly later than tl 

 person who produced it and is close to the apparatu 

 but he sees the rings approach quite leisurely, and 1 

 feels their impact if they strike his face, while the in 

 pact of sonorous waves is not felt unless the sound 

 of extreme loudness, as, for instance, the report 

 a discharged cannon. With such a ring the flame of 

 candle may be blown out at a distance of 2 m or 3 m , ai 

 at even a greater distance with a larger apparatus, if 

 portion of the circumference of the ring reaches t 

 wick ; but a much louder sound than that caused by t 

 knocking of the finger would have no effect whatev 

 upon the candle. The essential nature of the propag- 

 tion of sound is a series of successive condensations a 1 

 rarefactions of the air spreading out in all direction 

 whereby each individual particle of air is merely caus I 

 to move backwards and forwards through a small d- 

 tance; hence the objects upon which the oscillati 

 particles impinge are not perceptibly moved. But ; 

 the experiments with the smoke -ring apparatus 1 f - 

 whole mass of air which is propelled moves onward ; i 

 a definite direction, and its collision with bodies wlii> 

 are in its own path produces an effect which is co 

 paratively great. Thus a piece of paper suspended 



