xhausted, of
course the pendulum would experience the minimum of resistance and
would have to be lengthened a little. But in practice it is impossible
to secure the maintenance of a good vacuum without sealing up the case
in such a way as to render repairs very difficult, and this plan is
therefore rarely resorted to. What is usually done is to put the clock
in a metal case covered with a thick sheet of plate glass bedded in
india-rubber strips, and held down by an iron flanged lid or frame
firmly fixed by means of small bolts. An air-pump is attached to the
case, a turn-off tap being inserted, and by a few strokes the pressure
of the air inside the case can be lowered to (say) 29 in., or a little
below the usual barometric height at the place where the clock is. The
difference of pressure being small, the tendency of air from outside
to leak in is also small, and if the workmanship is good the inside
pressure will remain unaltered for many days. In any case the
difference produced by leakage will be small, and will not greatly
affect the going of the clock. With care, and a daily or weekly touch
of the pump, the pressure inside can be kept practically constant, and
hence the atmospheric error will be eliminated. The cover has also
incidentally the effect of keeping damp and fumes from the clock and
thus preserving it from rust, especially if a vessel with quicklime or
some hygroscopic material be put in the case.

Cases have considerable effect on the air, which moves with a pendulum
and is flung off from it at each vibration; the going rate of a
chronometer can be altered by removing the case. It is therefore
desirable that cases enclosing pendulums should be roomy. Many people
prefer to omit the air-tight case, and to keep a record of barometric,
thermometric and hygrometric changes, applying corrections based on
these to the times shown by the clock.

Suspension of pendulums.

It was formerly usual to suspend pendulums by means of a single spring
about 1/2 in. wide riveted with chops of metal. The upper chop had a pin
driven through it, which rested in grooves so as to allow the pendulum
to hang vertically. The best modern pendulums are now made with two
parallel springs put a little less than an inch apart. The edges of
the chops where the springs enter are slightly rounded so as to avoid
too sharp bending of the springs. Suspension of pe