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 band, it follows $$n$$N must be 300, in order to show the air effect, that is, the speed must rise to 6000 revolutions a minute with a light-journey of three times round. This speed would lead impractically near to the bursting-strength of materials; but it was hoped that, by taking the average of a series of settings, $$\tfrac{1}{200}$$th of a bend could be safely observed, and thus the effect of the air-spin detected. And if everything had gone well, I think this might have been done, but the difficulties met with caused a careful examination of the brick pillars end foundations beneath the floor, with the result of discovering that the brick pillars, by which the optical frame was ultimately supported on its gallows support (fig. 11, p. 767, 'Phil. Trans,' 1893), were not so entirely independent of the whirling machine's stone altar as they ought to have been. During the vacation bricklayers and carpenters were accordingly called in to re-set the warped pillars beneath the floor, and to clear away all joists end everything that could be suspected of in any way helping to transmit vibration. The result of this work was a beautiful steadiness end visibility at high speeds; but still the bands showed, what we call a concertina action, that is, a slight moving of the lateral bands in or out from the central one, as the speed varied. There would be no strong objection to such motion, if the precise symmetry of the central band could be assured, but, as this assurance was not forthcoming, the central band sometimes shared in this motion; and even if it did not, its steadiness was suspicious, because perhaps it ought to have slightly moved.

The residual trouble appeared possibly due to an obscure influence on the glass windows of the drum, possibly a slight warping due to warmth or air pressure, end an attempt was made to dispense with the window panes end to screen from the blast by another method. Accordingly a fresh drum was made of brass, with a pair of deflecting rims or flanges, so placed as to catch the air whirled off the disks all round, and deflect it out of the way upwards and downwards, the light passing on through a chink or slit in the brass drum to the region protected by these flanges (fig. 4).

Some residual draught did however manage to reach the mirrors, and, although they were strongly supported, it seemed to flutter them even if unable regularly to tilt them. It was then attempted to lessen the freedom of air supply to the axis of rotation, by wooden circular boards, fitting the axle loosely, and nearly as large as the drum, thus greatly interfering with the supply of sir. As soon however as the ventilation was thus interfered with the stir got distinctly hot, which was a worse evil.

The drum was supported separately on long wooden girders, so that no part of it