(C) will fit and slide in the
grooves and be held therein by the dove-tailed formation. Each block is
longer than the width of the groove, and has an outwardly projecting pin
which passes through a bar (D). One pin (E) is movable along in a slot,
but is adjustable at any point so that the shape of the ellipse may be
varied. The end of the bar has a series of holes (G) for a pencil, so
that the size of the ellipse may also be changed.

[Illustration: _Fig. 62. Bevel Gears_
_Fig. 63. Miter Gears_
_Fig. 64. Crown Wheel_
_Fig. 65. Grooved Friction Gears_
_Fig. 66. Valve_
_Fig. 67. Cone Pulleys_
_Fig. 68. Universal Joint_]

Fig. 70. ESCAPEMENTS.--Various forms of escapements may be made, but the
object of all is the same. The device is designed to permit a wheel to
move intermittingly or in a step by step movement, by the swinging
motion of a pendulum. Another thing is accomplished by it. The teeth of
the escapement are cut at such an angle that, as one of the teeth of the
escapement is released from one tooth of the escapement wheel, the
spring, or the weight of the clock, will cause one of the teeth of the
escapement wheel to engage the other tooth of the escapement, and give
the pendulum an impulse in the other direction. In the figure, A is the
escapement, B the escapement wheels and _a_, _b_, the pallets, which
are cut at suitable angles to actuate the pendulum.

Fig. 71. SIMPLE DEVICE TO PREVENT A WHEEL OR SHAFT FROM TURNING
BACK.--This is a substitute for a pawl and ratchet wheel. A is a drum or
a hollow wheel and B a pulley on a shaft, and this pulley turns loosely
with the drum (A). Four tangential slots (C) are cut into the perimeter
of the pulley (B), and in each is a hardened steel roller (D). It
matters not in what position the wheel (B) may be, at least two of the
rollers will always be in contact with the inside of the drum (A), and
thus cause the pulley and drum to turn together. On reversing the
direction of the pulley the rollers are immediately freed from binding
contact.

Fig. 72. RACKS AND PINIONS.--The object of this form of mechanism is to
provide a reciprocating, or back-and-forth motion, from a shaft which
turns continually in one direction. A is the rack and B a mutilated
gear. When the gear turns it moves the rack in one direction, because
the teeth of the gear engage the lower rack teeth, and when the rack has
moved to the end its teeth engage the teeth of the upper rack, thus
rever