76 PROFESSOR BARRETT, 
motion would seem impossible. But if it be a molecular motion 
one would expect the sound to be of a peculiar quality, the mole- 
cular motions of solids having a crepitating character. 
The solution of the difficulty seems to be this: Lord Rayleigh 
has shown (Nature, vol. 16, p. 114) that sonorous vibrations may be 
heard although the amplitude of the waves generating them be 
of transcendent smallness, an amplitude of one ten-millionth of 
a centimetre, Lord Rayleigh states is perfectly audible. Hence it 
is probable that the varied motions set up in the iron disc are 
motions of the disc as a whole, but of surpassing minuteness, and 
if so the telephone has directly confirmed Lord Rayleigh’s deduc- 
tions, and added to the wonders of our organs of hearing. 
The greatest obstacle to the practical introduction of the tele- 
phone is at present the disturbing noises produced by induction 
from powerful battery currents traversing neighbouring wires. 
Apart from this it would appear that conversation can easily be 
carried on two or three hundred miles apart, and breathing has 
been heard by Professor Bell at 150 miles distance. By employing 
a return wire instead of an earth connexion much of the inductive 
disturbance can be neutralized. In the trials of the telephone at 
the Royal Dublin Society, a loud ticking was heard every second 
which much interfered with telephonic conversation. This 
ticking was not due to any neighbouring clocks, but was found to 
arise from the general electric clock system of the town. Instead 
of a return wire connexion was made with the gas-pipes of the 
building; this “earth” happened to be the same as is employed 
in the electric clocks, and hence the disturbance. Substituting a 
second wire the disturbance was removed. 
Another obstacle at present before the telephone is the difficulty 
of its use in long submarine lines. This arises from the well- 
known cause termed “inductive embarrassment,” the line with 
its insulating sheath and the sea around acting as a condenser, 
and thus preventing the rapid delivery of electric pulsations. 
Nevertheless, conversation has very successfully been carried on 
between Dover and Calais and Holyhead and Dublin. The mere 
resistance of along line but little impedes telephonic conversation. 
The author of this paper has spoken with great ease through an 
actual line of some thirty miles, with an added resistance exceed- 
ing that of the Atlantic cable. The addition of the artificial 
