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 chamber) had insufficient area; the intensity of pressure was too great for lubrication, and accordingly the surfaces ground together and got hot, causing the oil vessel to smoke vigorously. Considerable power was, therefore, needed to drive it, even at 300 revolutions a minute, but at this speed some observations were made. The bands were so clear that $$\tfrac{1}{100}$$th of a band shift have been seen, but there was not a trace of shift when the mass was spinning, either with or without its magnetising current. The 110 volts of the town main were switched on and off and reversed many times, both when the mass was stationary and when it was revolving five times a second, but there was no effect.

Before proceeding to greater speeds it was obvious that the greater part of the weight of the iron mass must be taken off the pivot and dynamo-axle, and must be supported in some other way. A safety collar or guard attached to a frame above the wooden clutch, as shown in Plate 32, 'Phil. Trans.,' A, 1893, suggested the use of ball-bearings resting on this collar, which hitherto had been an inactive safety-guard, but was quite strong enough to support the weight required.

Accordingly we had this arrangement made, all the weight of the spheroid now rested on the guard collar by means of ball-bearings, and the steel pivot had nothing but the dynamo armature to support, this axle also being quite relieved from strain. The old wooden friction clutch was now of course useless, and it was replaced by gripping brass collars on the ends of the joining shafts, just below the ball-bearings, the power being transmitted by a pair of tangential stout copper wires, one on each side, looped round screw heads on the brass collars, so as to transmit a driving couple of considerable magnitude; but if by any accident the force transmitted was too great, the wires could snap and permit independent movement of the mass. Parenthetically it may be here stated that the wires broke several times during the course of the series of experiments, showing that the precaution was very necessary, and that a rigid connexion between the axles would have been dangerous.

During these alterations, other experiments, to be presently recorded, were in progress, and it was not till May, 1893, that careful optical spins were again conducted with the iron spheroid.

At this date the fringes were sometimes used after the light had been four or five times round, but usually the superior brightness and definition of the three-times-round beam was preferred. With a driving current of from 30 to 40 amperes and a voltage of about 80, the speed of 1000 a minute was readily maintained in the heavy mass by aid of the ball-bearings.

At first, however, a new disturbing phenomenon was observed: on beginning a spin the bands began to tremble and became partially obscure. This was not from shaking, nor did it seem likely to be due to wind reaching the semi-transparent plate, because the speed was quite slow. Screens glazed with microscope cover-glass were nevertheless provided, and next day another attempt was made. The flickering of the bands was just the same as before at low speeds, although there was no