632 Proceedings of Royal Society of Edinburgh. [sess. 
attached to the ether in contact with it firmly enough to prevent 
slipping. The circuital impulse on the ring by the generation 
of the magnetic field will give rise to a rapidly-subsiding train of 
waves of transverse vibration, of the kind which, in communica- 
tions to Section A of the British Association * at its meeting in 
Bristol last September, I described as a solitary wave of transverse 
vibration of the simplest possible kind in an elastic solid, and 
again, for periodic motion, as a very simple and symmetrical case 
of a train of periodic waves of transverse vibration. The work 
done by the circuital force on the ring is spent on waves of this 
class travelling outwards through ether, and in a very short time 
the ring comes practically to rest. It does not come to perfect 
rest suddenly by the departure from it of waves carrying away all 
its energy ; it subsides to absolute rest in an infinite time according 
to the law e^sin qt. The resinously-electrified fluid within the 
ring continues revolving with unaltered energy as long as the 
force of the magnetic field is maintained constant. 
§ 4. The simple molecular arrangement thus imagined supplies 
the rotatory or revolutional motion and the “ moment of momen- 
tum,” which forty-three years ago I pointed outf as wanted to 
explain, “simply by inertia and pressure,” the rotation of the 
plane of polarisation, then recently discovered by Faraday, for 
light transmitted through heavy glass in a powerful magnetic 
field along the lines of force. In my Baltimore Lectures I showed 
that embedded gyrostats would in fact produce exactly the rotation 
of the plane of polarisation in a magnetic field discovered by 
Faraday. The idea which forms the subject of the present 
communication shows how the fly-wheels of the gyrostats may he 
started into rotation in virtue of the generation of the magnetic 
field and stopped when the magnetic field is annulled. 
§ 5. The simply embedded gyrostat has not, however, the vibra- 
tional quality which is the essential of the Stokes-Maxwell- 
Sellmier vibratory molecule. For this a gyrostatic vibrator, 
capable of originating from a single blow on itself a suhsidential 
* “ On the Simplest Possible,” etc. : “On Continuity in Undulatory 
Theory,” etc., B. A. Report, 1898. 
t “ Dynamical Illustrations of the Magnetic and the Helicoidal Rotatory 
Effects of Transparent Bodies on Polarised Light.” — Proc. Boy. Soc. Bond ., 
vol. viii., June 1856 ; Phil. Mag., March 1857. 
