126 Transactions of the American Institute. 



again in the lantern ; now both its prongs have expanded, fan-like, 

 into an outline somewhat hazy, which you will, no doubt, recognize 

 as the result of their being in rapid motion ; but to make this yet 

 clearer, I will now bring a light pith ball in contact with one of them, 

 when, as you see, it will be violently knocked away whenever it 

 touches the moving fork. (See Fig. 4.) 



Let us take now another example. I have here a glass plate, sup- 

 ported at its middle, and by drawing the bow across its edge I make 

 it vibrate, and emit a bell-like sound. But you say again, perhaps, 

 let us see that this plate really vibrates. 



I reply, with great pleasure, and fix the plate in the vertical lantern, 

 so that one corner of it may come into the field. On this corner I 

 pour some water which is retained by a light rubber ring, and then 

 by means of the bow I again make the plate emit its fundamental 

 note. At once you see the screen covered with a beautiful pattern 

 of ripples, which express, as in legible words, the vibratory motion. 

 (See Fig. 5.) When, by a different management of the bow, the 

 plate is caused to emit a higher note, the pattern of waves at once 

 becomes finer, showing a quicker, and therefore shorter, wave 

 motion. 



I think one could hardly ask a clearer demonstration of the iden- 

 tity of sound and vibratory motion than this experiment affords. 

 But yet further, to clinch our argument, let us resort to another 

 experiment. We have here a brass rod firmly clamped at its middle, 

 and with an ivory ball so suspended as to rest against one of its 

 ends. (See Fig. 6.) Now, it seems hard to imagine that in order 

 to sound, when the hand, covered with rosin, is drawn -along it, this 

 bar must so vibrate, its particles moving away from and toward each 

 other, that it should actually grow longer and shorter, but yet 

 such is in fact the case ; for if I now draw my hand lightly along it, 

 you soon hear a shrill, metallic ring, and at the same moment the 

 ivory ball is violently driven away from the end of the bar. This 

 shows us that, improbable as it may seem, the bar does in fact 

 lengthen and shorten itself when in the act of emitting this sound. 



And now we will take one more illustration for the sake of a con 

 necting link, which it will furnish between one and another of the 

 vibratory forces. We have seen how the teeth of a saw, the teeth of 

 a file, or what may, by analogy, be called the teeth of the violin bow, 

 cause a vibratory motion when they are drawn across a body capable 

 of vibrating. It can, of course, make no difference if these teeth are 

 arranged about the periphery of a wheel, and the wheel being in 



