SENSITIVE FLAMES AND SOUND-SHADOWS. 



45 



Fig. 6. a, advancing sinusoid ; b, returning sinusoid ; 

 c, advancing and returning sinusoids, forming two 

 loops and a node ; c e is a whole wave-length ; c d, a 

 half wave-length. 



sound to be 1,120 feet, reducing this to inches, and dividing by 

 1*05 inch, the pitch of the whistle was thus found to be in the 

 neighborhood of thirteen thousand complete vibrations per sec- 

 ond. In no other way could this pitch be determined, for the 

 most accomplished musician loses his power of discriminating 

 pitch when either the upper or the lower limit of audition is ap- 

 proached. The pitch of the 

 highest tone employed in 

 music does not exceed five 

 thousand vibrations per 

 second. 



In performing this ex- 

 periment Lord Rayleigh 

 discovered an interesting 

 peculiarity of the human 

 ear in contrast with the 

 sensitive flame. By using 

 a tube, whose opening was 

 placed alternately in the 

 aerial loops and nodes, and 



conveying the sound thus to the ear at the same time that the flame 

 was alternately agitated and quiescent, he found that the ear was 

 most affected where the flame was least affected, and vice versa. 



The flame, moreover, is unequally sensitive in two directions 

 at right angles with each other. In drilling the small cylindrical 

 hole of the burner no amount of care is sufficient to prevent 

 minute irregularities. The current of issuing gas is not abso- 

 lutely cylindrical. It is disturbed slightly by interior currents 

 from side to side, and these affect the sensitiveness of the jet to 

 external disturbances. To test this, let the nozzle be rotated on its 

 own axis while the whistle is sounding, until the maximum effect 

 is noticed ; and let the sensitiveness of the flame be slightly re- 

 duced without causing it to cease to flare. On rotating the 

 nozzle now through a right angle the flame is found to become 

 quiet. Let a mirror be put on one side of the flame, a short dis- 

 tance off, so as to face the sensitive side. Adjusting it until it is 

 equally inclined to the directions of flame and whistle, the flaring 

 is started anew. This ceases when the mirror is rotated toward 

 either side through a very small angle. Indeed, no more beautiful 

 and exact illustration could be devised for showing the law of 

 reflection of sound-waves. The sound-ray, taking a longer and 

 broken path, disturbs the flame on its sensitive side, while the 

 direct rays are at the same time beating in vain against what by 

 analogy we may call its deaf side. 



Probably the most interesting acoustic phenomena to be in- 

 vestigated by the aid of the sensitive flame are those of diff rac- 



