_Disk or Disc._ A cylinder, whose length is very short in proportion to its diameter; a round plate with a hole in its center. _Double-eye or Knuckle Joint._ A joint formed of two forks or jaws with a cube of iron between them, with a bolt or pin through each jaw and the cube at right angles. Will work freely in all positions from a straight line up to 45°. _Double seated poppet valve._ A poppet valve having two valves on one stem, with two seats in the same shell. [Illustration: DOUBLE-SEATED POPPET VALVE.] _Drafting water._ Another term for “raising“ water by suction, in distinction to “forcing water.” _Drip-pipe._ A device used to draw off the water of condensation from systems of piping, steam cylinders, heaters, etc. Drain-cocks are used for similar purposes. “_Dutchman._” A piece “fitted in” to restore a worn part or to hide a defect. _Duct._ A passage or conduit. _“Duty” of pumps._ This indicates the measurement of the work performed by pumps. “Duty trials” are careful tests of the work done by the larger pumping-engines. _Elbow._ This fitting is used for uniting two pipes together at right angles. The illustration shows a malleable-iron gas-pipe elbow. [Illustration: ELBOW.] _Equilibrium Valve._ A valve balanced by an equal pressure on both ends. _Expansion Joint._ A telescopic slip joint having a packed stuffing box, permitting the parts it connects to expand and contract under variations of temperature. [Illustration: EXPANSION JOINT.] _Face._ The broadest flat surface of a piece of work, or the one having the greatest area. _Factor of Safety._ When a calculation of the ultimate strength of a machine is to be made it is necessary to provide for contingencies—this takes the form of a multiplier, and is called the factor of safety, or the margin of safety. _Feather, or sunk key._ A key that is fast in one piece of work, and an easy fit in the other, as a feather in a shaft. _Flow._ Motion of a fluid or liquid in one direction. “Flow-gate“ is a term sometimes applied to a riser. _Flume._ An open trough for conveying water. [Illustration: GATE VALVE.] [Illustration: GLOBE VALVE.] _Gate Valve._ A valve which opens the full area of the pipe, on the principle of a gate in a water flume. _Globe Valve._ A valve having a round ball-like shell as shown in the engraving. _Gland._ The sliding bushing for holding packing into a stuffing box, adjusted by studs and nuts. _Goose neck._ A pipe fitting having two bends in opposite directions which resemble the neck of a goose. _Gridiron Valve._ A type of slide valve familiarly called a “grid,” which may be circular or rectangular, consisting of alternate bars and spaces, sliding over a similarly formed seat, the object being to obtain the necessary steam way with a diminished amount of valve travel.