Per·cus·sion n.
 1. The act of percussing, or striking one body against another; forcible collision, esp. such as gives a sound or report.
 2. Hence: The effect of violent collision; vibratory shock; impression of sound on the ear.
    The thunderlike percussion of thy sounds.   --Shak.
 3. Med. The act of tapping or striking the surface of the body in order to learn the condition of the parts beneath by the sound emitted or the sensation imparted to the fingers.  Percussion is said to be immediate if the blow is directly upon the body; if some intervening substance, as a pleximeter, is, used, it is called mediate.
 Center of percussion. See under Center.
 Percussion bullet, a bullet containing a substance which is exploded by percussion; an explosive bullet.
 Percussion cap, a small copper cap or cup, containing fulminating powder, and used with a percussion lock to explode gunpowder.
 Percussion fuze. See under Fuze.
 Percussion lock, the lock of a gun that is fired by percussion upon fulminating powder.
 Percussion match, a match which ignites by percussion.
 Percussion powder, powder so composed as to ignite by slight percussion; fulminating powder.
 Percussion sieve, Percussion table, a machine for sorting ores by agitation in running water.

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 cen·ter n.
 1. A point equally distant from the extremities of a line, figure, or body, or from all parts of the circumference of a circle; the middle point or place.
 2. The middle or central portion of anything.
 3. A principal or important point of concentration; the nucleus around which things are gathered or to which they tend; an object of attention, action, or force; as, a center of attraction.
 4. The earth. [Obs.]
 5. Those members of a legislative assembly (as in France) who support the existing government. They sit in the middle of the legislative chamber, opposite the presiding officer, between the conservatives or monarchists, who sit on the right of the speaker, and the radicals or advanced republicans who occupy the seats on his left, See Right, and Left.
 6. Arch. A temporary structure upon which the materials of a vault or arch are supported in position until the work becomes self-supporting.
 7. Mech. (a) One of the two conical steel pins, in a lathe, etc., upon which the work is held, and about which it revolves. (b) A conical recess, or indentation, in the end of a shaft or other work, to receive the point of a center, on which the work can turn, as in a lathe.
 Note: ☞ In a lathe the live center is in the spindle of the head stock; the dead center is on the tail stock. Planer centers are stocks carrying centers, when the object to be planed must be turned on its axis.
 Center of an army, the body or troops occupying the place in the line between the wings.
 Center of a curve or Center of a surface Geom. (a) A point such that every line drawn through the point and terminated by the curve or surface is bisected at the point. (b) The fixed point of reference in polar coordinates. See Coordinates.
 Center of curvature of a curve Geom., the center of that circle which has at any given point of the curve closer contact with the curve than has any other circle whatever. See Circle.
 Center of a fleet, the division or column between the van and rear, or between the weather division and the lee.
 Center of gravity Mech., that point of a body about which all its parts can be balanced, or which being supported, the whole body will remain at rest, though acted upon by gravity.
 Center of gyration Mech., that point in a rotating body at which the whole mass might be concentrated (theoretically) without altering the resistance of the intertia of the body to angular acceleration or retardation.
 Center of inertia Mech., the center of gravity of a body or system of bodies.
 Center of motion, the point which remains at rest, while all the other parts of a body move round it.
 Center of oscillation, the point at which, if the whole matter of a suspended body were collected, the time of oscillation would be the same as it is in the actual form and state of the body.
 Center of percussion, that point in a body moving about a fixed axis at which it may strike an obstacle without communicating a shock to the axis.
 Center of pressure Hydros., that point in a surface pressed by a fluid, at which, if a force equal to the whole pressure and in the same line be applied in a contrary direction, it will balance or counteract the whole pressure of the fluid.

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