of Edinburgh, Sessio?i 1877-78. 
617 
the receiving disc the particles may not set up an action on 
their own account, independently of the displacement caused by 
the poles of the adjoining magnet. In a mechanical telephone, 
we do not find that it is made to sound only by the normal 
push or pull of the thread, the faintest rubbing on the irregu- 
larities of its surface, either on the disc or the tube to which it is 
attached, makes a sound loud enough to be heard, and we can easily 
admit that if an internal vibratory disturbance be set up in any 
direction in it, the same would be audible enough. In a discussion 
as to mechanical and molecular sounds, it may be safely admitted, 
where electricity or magnetism is concerned, that any action that is 
clearly traceable to disturbance within a body is molecular in its 
origin. It will, moreover, be granted that the mere smallness of any 
vibration does not necessarily give any clue to its origin. Infini- 
tesimal vibrations are not necessarily molecular, nor are vibrations of 
molecular source free from external motion ; and we can only say 
that a vibration comes from molecules if we can assign to it no out- 
side cause. It may, however, be to the point that a vibration may 
be assumed to be molecular because of the difficulty in suppressing 
it, a vibration springing from within being more independent of 
direction than one produced from without from one quarter. 
I propose in this communication to raise such questions in regard 
to the telephone, and though the results obtained may not be de- 
cisive, they may be some little contribution to the discussion. 
I would begin with a case where internal action seems wholly 
absent. I refer to the action of a tuning-fork on the telephone. 
It has been mentioned in more than one communication to the 
Society, that a tuning-fork acts best without the disc. We find that 
the loudest sounds are sent to the listener at the receiving telephone, 
when one prong is brought with its flat vibrating end in front of 
the core or pole pin, and next to that when the prongs, if they are 
not too far apart, are laid with their flat sides vertical at an equal 
distance on each side of the pin. When the handle of the fork is 
laid on the core, and held upright, the resonance of the wooden 
frame of the telephone and the table on which it rests becomes loud, 
but only a faint trace of this is sent to the distant hearer. If we 
magnetise two like forks, one which we may call A, to be like a bar 
magnet having the end of the handle as one pole, and the other 
