230 
experimental confirmation of the helical character of the 
magnetic wave. The energy or, as it has sometimes been 
erroneously termed, the inertia of rotation — i.e., the resistance 
which a rotating body offers to any change in the direction of 
its axis of rotation — is well exemplified by the gyrascope, and 
a more familiar illustration is found in the undeviating path of 
the rifle-ball. Now, if a mass of copper be suspended by a 
string between the poles of a powerful electro-magnet, and be 
put in rapid rotation by twisting the string, the instant that 
the magnet is excited, the rotation is arrested ; and if the mass 
be now forcibly rotated, so much heat is developed by molecular 
friction, that fusible metal contained in a copper tube similarly 
placed may be actually melted and poured out. This arrest of 
the motion of the rotating mass would be a necessary dynamical 
sequence of the helical wave-motion assumed to constitute 
magnetic energy ; for in that case each disturbed molecule 
would be describing a circular orbit in a plane at right angles 
to the lines of magnetic energy, and would by its own energy 
resist any displacement of its axis of revolution; and this view 
maybe further confirmed by another experiment. A ball of copper 
with a small pulley on its axis is placed at the enil of a frame, 
so as to be capable of being rapidly rotated by a wheel and band, 
when placed between the poles of the electro-magnet ; its axis 
of rotation either coinciding with, or being placed at right 
angles to, the lines of magnetic energy. When the axis of 
rotation of the ball coincides with the magnetic lines, there 
would be obviously no change in the direction of the planes of 
the assumed molecular revolution, and consequently no heat 
ought to be developed ; this may be shown to be the case by 
means of a thermopile connected with a galvanometer and 
brought near the rotating ball. When, however, the axis of 
rotation of the ball is placed at right angles to the magnetic 
lines, heat will be immediately developed. 
36. The gyratory nature of the magnetic wave is further con- 
firmed by a fact first observed by Earaday — namely, that if a 
beam of polarised light be transmitted through a piece of heavy 
glass placed between the poles of an electro-magnet, so that 
the axis of the beam may correspond with the lines of magnetic 
energy, then, if the magnet be excited, the plane of polarisa- 
tion is twisted a little, either to the right or left, according 
to the direction of magnetic polarity — a result by no 
means inconsistent with the hypothesis of molecular revolu- 
tion. 
37. In both the essays before alluded to, the Ether” theory 
has been put forward as evidence of the divergence of opinion 
existing between physicists. It must, however, be observed that 
