358 Correspondence. 
dynamical consequences of the above assumed condition of 
the extra currents. The initial extra-current will excite a 
similar impulsive motion in secondary coil, just as one 
stretched chord will excite another capable of vibrating in 
unison with it ( for electro-dynamic induction is probably 
quite analogous to the reciprocation of sound), and the re- 
coil of this impulse (the zzztzal induced current) will be 
weakened in opposing the continuous motion induced by the 
continuous primary current. Again, the recoil of the im- 
pulse induced in the secondary coil by the terminal extra 
current in its primary (the terminal induced current), will 
likewise be in a direction contrary to that of the inducing 
impulse, but its potential will remain undiminished, as it is 
un-opposed by any continuous induction; and it will, 
therefore, possess greater force than the initial induced 
current. 
But it may be asked by those who find a difficulty in 
dispossessing their minds of long-established and time- 
honoured notions. Is not all this about “ inertia,’ and 
“vis-viva,’ and “impulses,’ sheer hypothesis ? Not so: 
The writer has long since observed and recorded a precisely 
similar phenomenon (apparently an exact analogue of the 
initial extra current) in an unquestioned case of wave 
motion, an experiment on the interference of sound waves, 
due in common with so many others to the genius of Pro- 
fessor Wheatstone. Let the handle of a vibrating tuning- 
fork, held obliquely, rest on the surface of a table. As 
long as it remains at rest, a loud resonance of the table is 
audible: but if the tuning-fork be moved parallel to itself 
along the surface of the table in any direction, the reso- 
nance of the table immediately ceases, from the perpetual 
interference of the vibrations in successive parallel planes 
with each other. The instant the tuning-fork stops, the 
resonances burst out again in a very striking manner If 
the tuning-fork be held vertically, the planes of vibration 
coincide, and the resonance is not interrupted by moving it. 
It can scarcely be doubted that the impulsive recommence- 
ment of the resonance after its interruption by interference, 
is entirely due to the dynamical cause here assigned to the 
initial extra current. 
M. Chauveau (no doubt correctly) ascribes the phyatiatia! 
gical effect of an electric impulse to the dynamical moleculor 
disturbance which it produces. 
It will naturally be asked if electricity be only wave 
motion, what is the nature of that motion, and in what re- 
ae — 
