Scientific Lectures. 125 



by Professor Pierce, Superintendent of the United States Coast Sur- 

 vey.; If we compare the range of our sound perceptions with those 

 of heat and light, and the interval between the first and the others, 

 we find that while the range of sound embraces some twelve octaves, 

 that of heat and light includes between one and two octaves for 

 each (by an octave I mean the range included between any given 

 number of vibrations in a second, and double that number), while 

 between the highest audible note and the lowest heat rate which we 

 can estimate, comes a possible series of some thirty octaves of rates 

 of vibrations inappreciable by any of the senses which we now pos- 

 sess, but capable of furnishing causes of perception to six or eight 

 senses which might be fitted to respond to their excitations. 



To return, however, to our assertion that sound, heat and light are 

 all vibratory motions, increasing relatively in rate in the order in 

 which we have named them. This I desire to prove to you by a 

 series of experiments conducted before your eyes this evening. 



First, then, to prove thaft sound is merely a vibratory motion, I 

 have here a strip of metal, fastened upright in a vise, and carrying a 

 large circle of card on its end, and I hold in my hand a carpenter's 

 saw, with large teeth ; if I draw the saw slowly over the strip of 

 metal near where it is held in the vise, you will see the card wave 

 back and forward as each tooth passes. (See Fig. 3.) 



Here we have a vibratory motion, but one too slow to produce 

 sound ; for that purpose, we must have motions at least as rapid as 

 sixteen full back-and-forth movements in a second. To accomplish 

 this, we substitute for the elastic rod, which cannot move quickly, a 

 stiff piece of wood, and for the saw, with its teeth far apart, a file, 

 with close teeth ; then we get just a similar motion to the former, 

 but quicker ; each tooth of the file catches the wood, pulls it a little 

 way and lets go, as did the saw, and we then have, as you perceive, 

 a very manifest, if disagreeable, production of sound. 



Or, again, I take this piece of metal, called a tuning fork, and draw 

 across it this violin bow (which is only a sort of musical file), made 

 rough with rosin and the natural asperities of the hair, and from it 

 proceeds a far more agreeable description of sound. 



But, you may ask, how do we know that the fork is really moving ? 

 We cannot see its motion as we did that of the elastic rod. True, 

 but you shall see it ! I now place this tuning fork, as an object, in 

 the lantern, and you see its enlarged image on the screen, the prongs 

 being perfectly straight or of uniform thickness; I now sound it with 

 the bow, as before, and while you hear its musical voice, I place it 



