240 Proceediixjs of the Royal Irish Academy. 
immediately pressed on the belly of a violin, the effect of which in 
ordinary cases is considerable and well known. 
So far as the experiment goes, it shows that the two prongs act 
in concert ; for if not, and if they sounded independently of each other, 
the ring of one should not have been extinguished by the dipping of 
the other. The same inference is more strongly suggested by the fol- 
lowing. An invention was registered some years since for a tuning- 
fork furnished with a sliding weight on each of its prongs, which, by 
being moved up or down, both together, altered the pitch of the ring, 
and aiforded a succession of notes of the tempered diatonic scale. The 
sliding weights should be relatively in the same position on the prongs. 
If one weight were moved a little down, the pitch was sharpened ; 
but, if moved lower, the fork when struck would no longer give any 
sound ; for then that prong could not vibrate, and the other would not, 
as both were not in condition to do so ; proving that the prongs do 
not give independent vibrations, but that the instrument acts as a 
whole. 
In support of his opinion Earl Stanhope states that it is fre- 
quently practicable to get rid of the beating in a tuning-fork by very 
carefully filing the two prongs so as to make them exactly alike 
throughout." Doubting the necessity, as well as the alleged efficacy, 
of filing, I tuned a piano-forte string precisely to a fork which gave 
the note C, and filed off about a quarter of an inch from the length of 
one of the prongs, thus creating a dissimilarity which could never 
occur to this amount in the manufacture. This should have caused a 
beat that could be easily appreciated by the most inattentive ; the 
effect was, that the fork now gave a clear ring, a semitone sharper than 
the piano, showing, as in the former case, that it vibrated as a whole ; 
there was no beat ; thus proving that inequality could not cause the 
imperfection attributed to it. 
The connexion between the teeth and the organ of hearing is so well 
known that it is common to say of any grating sound that ^' it sets the 
teeth on edge." Dr. Brewster quotes from Chladni that two persons, 
who had stopped their ears, could converse with each other when they 
held a long stick, or a series of sticks, between their teeth. The sound 
could also be heard when a thread was held between the teeth by 
both, so as to be somewhat stretched." A French commission was 
appointed by the IS'atibnal Institute, in 1800, to examine a discovery 
made by Citizen Yidron, a music-master at Paris, for rendering music 
audible by persons deaf and dumb from their birth ; it consisted in 
taking a steel rod between the teeth, the other end of which is placed 
on the musical instrument" (Nicholson's Journal, iv., 383). 
The vulgar experiment of holding a steel poker to the teeth, while 
it is made to vibrate by a stroke, suggested to me the probability 
that by holding the tuning-fork to my teeth, the sound would be so 
much intensified that a beat in its ring, if it exist, might be percep- 
tible, although not discoverable under ordinary circumstances. 
On making the trial to different teeth, I found that the fork 
