harmonic, and it is an
analytical principle that harmonic motions superposed on one another
still remain harmonic. Hence, then, the action of a spring superadded
upon the action of gravity on a pendulum still leaves the motion
harmonic. But changes of temperature would affect the spring
considerably. In the case of such a spring the repose faces of
Graham's escapement might be minimized and the escapement checked each
side by a stop, so as to prevent the pallets from rubbing on the
points of the scape-wheel. Graham's escapement can, if well made, be
arranged so as not to vary more than an average of 1/30 of a second
from its mean daily rate, and this is so good a result that many
people doubt whether further effort in the direction of inventing new
escapements will result in any better form. Two adaptations of
Graham's escapement have been made, one by Clemens Riefler of
Nesselwang, and the other by L. Strasser of Glashutte, Saxony, which
give good results in practice. Riefler's scheme is to mount the upper
block, into which the suspension spring is fastened, upon knife edges,
and rock it to and fro by the action of a modified Graham's
escapement, thus giving impulses to the pendulum. Fig. 11 shows the
arrangement. PP are the agates upon which the knife edges CC rest. A
is the anchor, RH the scape-wheels, and S the pallets.

Strasser's clock is arranged on the same idea as that of Riefler, only
that the rocking motion is given, not to the springs that carry the
pendulum, but to a second pair of springs placed outside of them and
parallel to them. The weight of the pendulum is therefore carried by
an upper stationary block, but above that a second block is subjected
to the rocking motion of the anchor. The general design is shown in
fig. 12. The pallets are each formed of two stones, so contrived as to
minimize the banging of the teeth of the scape-wheel. Both Riefler's
and Strasser's clocks aim at haying a virtually free pendulum; in
fact, they are in reality adaptations of the principle of the
spring-clutch to Graham's escapement. The weak point in both is the
tampering with the suspension.

Detached escapement.

The dead escapement is not, however, truly free. In order to make a
free escapement it would be necessary to provide that as soon as the
pendulum approached its centre position, some pin or projecting point
upon it should free the