_Main._ A principal pipe or duct as distinguished from lesser ones, especially a principal pipe leading to or from a reservoir or a fire-main; a “forcing main” is the delivery pipe of a pump. _Mean gradient._ The grade of a pipe-line which should be made as nearly straight as possible to avoid air pockets. _Miner’s Inch._ The amount of water that will flow per minute through an opening one inch square in a plank two inches thick, under a head of four inches of water above the upper edge of the opening, and is equal to nine United States gallons. _Mississippi River gauge cock._ A cock without a handle or thread upon the stem and designed to be opened by pressure upon the top end of the stem as shown. [Illustration: MISSISSIPPI RIVER GAUGE COCK.] _“Modulus” of a steam pump._ The _measure_ or multiplier of power used in operating pumps. _Modulus_ has nearly the same meaning as measure. _Nipple._ A short connecting piece of pipe threaded upon both ends. [Illustration: NIPPLE.] _Outboard delivery pipe._ The pipe which leads, in steam vessels, from the condenser through the side of the ship. _Pipette._ A small tube used to withdraw and transfer fluids or gases from one vessel to another. The shape differs with the special use to which it is adapted: some are graduated to measure fluids accurately as well as to transfer them. _Penstock._ The barrel of a pump in which the piston plays and through which the water presses up; also the conduit or trough from the source of supply to a water wheel. _Pet-cock._ This is an air-cock. See air-cock. _Pipe-clamp._ A device for connecting one pipe to another without cutting the pipe and inserting a tee; a _pipe-saddle_ performs the same office as the above, but for larger pipes. _Pitcher Pump._ A hand pump which takes its name from the shape of its discharge. [Illustration: PITCHER PUMP.] _Plug-valve._ This is a tapering plug which turns in a shell, example, the _plug of a faucet_. See Cock. A _fire-plug_ is a street hydrant to which a hose may be attached. [Illustration: PLUMB BOB.] _Plumb-bob._ This is a device for testing whether anything stands exactly vertical; a _plumb-rule_ contains a plumb-bob. _Pressure-reducing Valve._ A valve for reducing high boiler pressure to low pressure, for steam heating, etc. [Illustration: PRESSURE REDUCING VALVE.] [Illustration: SECTION.] _Priming._ To fill a pump with water when it refuses to lift of its own action, is called “priming the pump.” _Pump-brake._ The handle or lever by which a pump is worked. _Pump-box._ A cap or case covering the top of a pump; the casings of the upper and lower valves are the _upper_ and _lower_ pump boxes. _Pump-chain._ An endless chain with discs forming valves at proper distances, working on two wheels, one above and one below, and passing down outside and returning upward through a wooden tube like a belt.