Scientific Lectures. 195 



A simple arrangement of the kind indicated would transmit to the 

 brain merely a general sensation of sound, just as the blind, on turn- 

 ing their eyes to the sun, have a general sensation of light ; but what 

 is the apparatus which enables us to distinguish between different 

 sounds, high and low notes, and notes'of different quality? Let us 

 place this question distinctly before ourselves ; but, before answering 

 it, allow me to make a couple of experiments that have a bearing on 

 its solution. We have here a tall gas flame, issuing from a beautifully 

 prepared jet (which was made and given to me years ago by Profes- 

 sor Mayer, of Hoboken), and I now arrange the pressure on the gas 

 so that the flame is almost ready to flare or vibrate, of course at a rate 

 of its own selection ; and if I send to it sound vibration of its own 

 chosen kind, it will instantly acknowledge the relationship ; it will 

 shorten itself, flare and roar. It pays, for example, no attention to the 

 sound of this two-feet organ-pipe, but, as you see, instantly responds 

 to the least sound from the bunch of keys. I pronounce to it the 

 vowel sounds ; it is evidently deaf to the German TJ, does not pay 

 much attention to O, more to E, I, and much more to the sound Ah. 

 As you notice, the least hiss, even from a distance, causes it to shorten 

 and roar. So we see, in a general way, that a stream of gas can be 

 set in vibration by the sound-waves which it has a tendency itself to 

 generate. 



But, still more accurately to the point is an experiment with two 

 similar tuning-forks, which I now wish to make. There are two tun- 

 ing-forks, in all respects alike, mounted on their boxes, and exactly 

 in unison. When I set one of them in action, the feeble sound 

 vibrations travel through the air and set the second fork, with all its 

 mass of heavy steel, also in vibration, simply because it naturally 

 vibrates at exactly the same rate, for, owing to this fact, the trans- 

 mitted vibrations always reach it at the right instant for co-operation, 

 all conflict being avoided. But, in this large house, the greater num- 

 ber of you would not be able to hear the feeble vibrations of the 

 second fork, and I have caused this delicately suspended mirror, which, 

 hanging by a hair that is fastened above and below, can be set in 

 motion by the least vibration of the fork in contact with it. The 

 mirror reflects a beam of light on the screen ; it is now stationary ; 

 but the instant the bow is drawn over its companion, it begins to 

 move, and the motion becomes each moment more violent, till now 

 the spot of light sweeps over the whole screen, and you notice that, 

 even at a distance of several feet, we have the same result repro- 

 duced. By the use of a single silk fiber, instead of a hair, the deli- 



