in the sockets, the speed is very much diminished, and the pivots begin to
wear. In Foucault's apparatus oil was furnished to the pivots, through
small holes running through the screws, by pressure of a column of
mercury. In this apparatus it was found sufficient to touch the pivots
occasionally with a drop of oil.

[Illustration: FIG. 7.]

Fig. 7 is a view of the turbine, box, and supply-tube, from above. The
quantity of air entering could be regulated by a valve to which was
attached a cord leading to the observer's table.

The instrument was mounted on a brick pier.



The Micrometer.


[Illustration: FIG. 8.]

The apparatus for measuring the deflection was made by Grunow, of New
York.

This instrument is shown in perspective in Fig. 8, and in plan by Fig. 9.
The adjustable slit S is clamped to the frame F. A long millimeter-screw,
not shown in Fig. 8, terminating in the divided head D, moves the carriage
C, which supports the eye-piece E. The frame is furnished with a brass
scale at F for counting revolutions, the head counting hundredths. The
eye-piece consists of a single achromatic lens, whose focal length is