1898-99.] Lord Kelvin On Magnetism. 633 
train of at least 200,000 waves of light, must be connected with 
the surrounding ether by springs, having sufficient resilience to 
store up in themselves the total energy thus radiated out. Taking 
now as gyrostat our electric doublet of vitreously-electrified rigid 
hollow ring filled with fluid resinously electrified, consider what 
must be the nature of the elastic communication between it and a 
rigid lining of a spherical hollow in ether around it, to fulfil some 
of the known conditions of radiant molecules. 
§ 6. (a.) Let the spring connection be equivalent to a simple 
force between I, the centre of inertia of ring and fluid, and O, the 
centre of the spherical sheath, varying directly as the distance 
between those points. The gyrostatic influence will be inoperative, 
and the result will be precisely the same as if we had a single 
Maxwell-Sellmier material point at I, of mass equal to that of ring 
and fluid together. 
( b .) Let points on the ring be connected by springs with points 
on the sheath. Supposing now the sheath to be held fixed, the 
stiffnesses and the tensions of these springs may be adjusted to 
give 21 arbitrary values for the co-efficients in the quadratic for 
the potential energy of any infinitesimal displacement, specified 
by three components of linear displacement of I, and three com- 
ponents of rotational displacement round axes through I. The 
well-known solution of the problem of infinitesimal vibrations 
about a position of equilibrium of a rigid body, modified in respect 
to moments of inertia to take into account the fluidity of the 
incompressible fluid in the ring, gives us immediately the periods 
and geometrical specifications of six fundamental modes of simple 
harmonic vibration. Hence our combination, serving as a radiant 
molecule, without magnetic force would give six bright lines 
(understood of course that each of the six periods is within the 
range of light-periods). Suppose now a vast number of such 
molecules, all equal and similar in every respect but with different 
orientations, to be scattered through a flame. Each molecule, 
whatever its orientation, will give six lines of the same periods, 
though of different intensities when seen in any particular direc- 
tion, according to the chances of orientation and of impulses.' 
Hence each of the six bright lines will be perfectly sharp. 
§ 7. How suppose a magnetic field to be suddenly instituted. 
VOL. XXII. 10/11/99 2 S 
