*Radius.* That line from a center to the circumferential or outer line or point. *Rectilinear.* Right; straight. *Recessed.* A depression; a cavity. *Reinforce.* The term applied to any means which may be employed to strengthen any part. *Requisite.* Necessary. *Registering.* To indicate; also applied to mechanism where one part exactly coincides with another part. *Resorted.* Where one device is utilized instead of another. *Refinement.* Made better; a more satisfactory form. *Retreating.* Falling back; one part to the rear of another. *Reversed.* Turned about; in the opposite direction. Inversed means; upside down. *Resiliency.* That property of matter which will yield, or change its form and return to its original form. *Resistance.* That property of all matter to object to a change of form; the force which opposes the movement of a current through a conductor. *Ribbed.* Having raised surfaces, of greater or less length. *Rock shaft.* A shaft which turns back and forth; oscillating. *Rotatable.* Turning, as a wheel on its axle; often confounded with the word revoluble, which describes the movement of a body through an orbit. The earth rotates on its axis once every twenty-four hours; it revolves through space around the sun once every year. *Rotor arm.* An arm adapted to swing through an arc. *Saturate.* To absorb a liquid or gas to full capacity. *Secondary.* The second place; applied to a battery, it is one which receives and holds a charge. *Sectional.* A part; a view across the parts. *Selector.* A plate with notches in it for a lever, which is moved through the openings at the will of the operator. *Selector plate.* The notched, or slotted plate, in which the transmission lever travels.