thick. It was silvered on one side only, the reflection taking place from
the outer or front surface. A species of turbine wheel, T, is held on the
axle by friction. This wheel has six openings for the escape of air; a
section of one of them is represented in Fig 6.

[Illustration: FIG. 5.]

[Illustration: FIG. 6.]



Adjustment of the Revolving Mirror.


The air entering on one side at O, Fig. 5, acquires a rotary motion in the
box B, B, carrying the wheel with it, and this motion is assisted by the
reaction of the air in escaping. The disc C serves the purpose of bringing
the center of gravity in the axis of rotation. This was done, following
Foucault's plan, by allowing the pivots to rest on two inclined planes of
glass, allowing the arrangement to come to rest, and filing away the
lowest part of the disc; trying again, and so on, till it would rest in
indifferent equilibrium. The part corresponding to C, in Foucault's
apparatus, was furnished with three vertical screws, by moving which the
axis of figure was brought into coincidence with the axis of rotation.
This adjustment was very troublesome. Fortunately, in this apparatus it
was found to be unnecessary.

When the adjustment is perfect the apparatus revolves without giving any
sound, and when this is accomplished, the motion is regular and the speed
great. A slight deviation causes a sound due to the rattling of the pivots