_Hand-Nut._ A nut having wings or projections so that it may be screwed up by hand without the aid of a wrench. [Illustration: HAND-NUT.] _Head of water._ In hydraulics “head“ means pressure due to height of column of water. _Heat Units._ The unit of heat is the amount of heat required to raise one pound of water one degree, usually from 32° to 33° Fahr. “_Hesitates._” A pump is said to “hesitate” when the motion becomes uncertain. _Horse-power of a pump._ Is the same as is used to designate that of a steam engine, with this exception: the initial pressure in the pump remains constant throughout the stroke. Formula is the same as for a steam engine. “_Hump._” This is an arch or bend which causes an “air pocket” in a water-pipe line. _Hydrant._ A valve and spout connecting with a street main. _Hydraulic Belt._ An endless woolen band for raising water. The lower bight is immersed in water, and the upper bight passes over a roller. The belt travels about 1,000 feet per minute, and discharges at its upper turn. _Hydraulic Jack._ A lifting device in which a ram, a pump, and liquid is used instead of a screw. [Illustration: HYDRAULIC JACK.] _Hydraulic Pivot._ A “slippery liquid support” for an upright shaft, a film of water being introduced beneath it by pressure to support the weight thereof and prevent the usual friction of the shaft on its step. _Hydraulic Shears._ A machine for shearing or cutting metals, etc., by the force of water pressure operating cutters. _Hydraulic Valve._ A valve for regulating the distribution of water in the cylinders of hydraulic elevators, cranes and other water pressure machines and devices. _Hydraulic Wheel._ One for raising water by applied power, as the Noria Scoop wheel, tympanum, etc. See illustrations in section relating to the history of the pump. _Impact._ The single instantaneous shock of a body in motion when it strikes against another body either in motion or at rest. _Leakage._ The loss of water from any cause. “_Lift and drop of a valve._” This term indicates the amount of “play” up and down, designed to be given to a valve by its designer. _Liner._ A piece of iron or other metal put behind or on a piece to take up its wear. _Lost Motion._ Motion that is not transmitted on account of the looseness of the parts, hence it is lost. “_Losing water._” A term used when the pump stops, caused by air leaking into the suction pipe, or foreign matter clogging the strainer at the end of the suction pipe. _Low pressure steam._ Steam which is either below 30 lbs., or but a few pounds in excess of the atmospheric pressure. _Lug._ That which projects like an ear, especially that by which anything is supported, or against which anything bears, or through which a belt passes.