REFLECTION OF SOUND. 4S 



will now start his reed and regulate the sensitiveness of his flame. 

 The flame, you observe, is steadied by the interposition of my hand 

 between it and the tube. And now, instead of my hand, my assistant 

 turns the series of gas flames round, the heated columns of air enter 

 the tube, and that moment the flame is stilled as effectually as when 

 I intercepted the waves with my solid hand. This experiment illus- 

 trates all of a similar class. We have operated with a great number 

 of gases. We have here, and can use, if necessary, carbonic acid gas, 

 hydrogen gas anything, in fact, that will render the column of air 

 non-homogeneous. I said, subsequently, to my assistant that we 

 ought to be able, not only to shew the interposition of the sound 

 waves by these different layers of gaseous matter, but by proper 

 experiment to be able to make the echoes evident by their action 

 upon a flame. Mr. Cotterell, in an exceedingly ingenious and beau- 

 tiful manner, devised the means of doing so, and he will now make 

 his experiment himself. Two tubes are placed so as to form a V; at 

 the end of one of them is placed a vibrating reed, and opposite the 

 end of the other a sensitive flame. The two ends here referred to are 

 those widest apart. The sound from the reed passes down one tube, 

 impinges on the broad flame of a batswing burner, and is reflected 

 by it through the other tube to the sensitive flame, which is thrown 

 into violent agitation by the waves reflected from the hot gaseous 

 layer. As to the transmission of these sounds through different media, 

 it is very desirable that we should be clear upon this subject, inasmuch 

 as various theoretical notions have been entertained which had a very 

 important practical bearing as regards the establishment of sound 

 signals at sea. It was necessary to test these notions with the utmost 

 strictness, to examine the permeability of various kinds of air and 

 various states of the atmosphere for the waves of sound. Here is a 

 vibrating reed, from which the waves of sound will pass through the 

 tube, and act upon the flame. The interposition of my hand com- 

 pletely stops the action, and causes the flame to become quiescent. 

 I will next try a pocket-handkerchief. The sound waves go through 

 the handkerchief as if it were not there. I take a flannel, and you 

 will observe that it has hardly any influence on the sound. I double 

 it so as to make a thick blanket of four layers, and that is practi- 

 cally transparent to the waves of sound. Here is a thick woollen 



