ndulums on knife
edges was tried by B. L. Vulliamy and others, but did not prove a
success.

It was once thought that lenticular pendulum bobs resisted the air
less than those of other shapes, but it was forgotten that their large
surface offered more "skin friction." They are now no longer used, nor
are spheres on account of difficulty of construction. A cylinder is
the best form of bob; it is sometimes rounded at the top and bottom.

_Escapements._--The term escapement is applied to any arrangement by
which, as the wheels rotate, periodic impulses are given to the
pendulum, while at the same time the motion of the wheels is arrested
until the vibration of the pendulum has been completed. It thus serves
as a mechanism for both counting and impelling. Since the vibrations of
a pendulum through small arcs are performed in times independent of the
length of the arc, it follows that if a pendulum hanging at rest receive
an impulse it will swing out and in again, and the time of its excursion
outwards and of its return will remain the same whatever (within limits)
be the arc of the swing, and whatever be the impulse given to it. If the
impulse is big, it starts with a high velocity, but makes a larger
excursion outwards, and the distance it has to travel counteracts its
increase of speed, so that its time remains the same. Hence a pendulum,
if free to swing outwards and in again, without impediment, will adapt
the length of its swing to the impulse it has received, and any
interference with it, as by the locking or unlocking of the escapement,
will be far less deleterious to its isochronism when such interference
occurs at the middle of its path rather than at the ends. It follows
that the best escapement will be one which gives an impulse to the
pendulum for a short period at the lowest point of its path, and then
leaves it quite free to move as it chooses until the time comes for the
next impulse.

But a pendulum is not quite truly isochronous, and has its time slightly
affected by an increase of its arc; it is therefore desirable that the
impulses given to it shall always be equal. If the escapement forms the
termination of a clock-train impelled by a weight, the driving force of
the escapement is apt to vary according to the friction of the wheels,
while every change in temperature causes a difference in the thickness
of the oil. It is therefore desirable, if possible, to secure uniformity
of impulse--s