_Union._ A fitting designed to unite the two screwed ends of a pipe, with a single nut to secure them. _Vacuum._ A void space; an inclosed chamber from which the air (or other gas) has been very nearly removed, as by an air pump. _Valve._ Any device or appliance used to control the flow of a liquid, vapor, or gas, or loose material, through a pipe, outlet or inlet; the term includes air, gas, steam and water-cocks of all kinds; water-gates, air-gates, etc. One hundred and fifty of such devices are named by Knight in his “Mechanical Dictionary.” _Viscosity._ Glutinous, adhering, or sticky, as tar, gums, molasses. Internal friction or resistance to change of shape. ^V^ _thread._ A thread on a rod or bolt cut in the form of a letter ^V^. _Washer._ A circular piece of leather, rubber, metal, or other material with a hole in its center, through which a rod or bolt may pass. _Water Arch._ A chamber of plates or of pipes over the furnace door of brick set boilers to take the place of the usual cast iron or fire brick arch, and connected with the boiler to supply it with water. The feed water is often introduced through the water arch. _Water-bellows._ A form of pump, like a bellows—of great antiquity. _Water-cap._ The cover for discharge valves on a steam pump. _Water-end._ The pump end of a steam-pump; in distinction from the steam end. _Water-hammer._ A noise caused by the pulsative motion of water inside a steam pipe, resembling the blows of a hammer. _Water Ram._ A hydraulic ram. _Working Barrel._ The water end of a pump. _Whirlpool-chamber._ A chamber attached to the discharge end of the centrifugal pump in which the whirling water gradually loses its rotation, thereby reducing friction. _Wing-nut._ An iron nut having a wing at each side. Sometimes called a “butterfly nut.” [Illustration: YOKE.] _Yoke._ A branch pipe, or a two-way coupling for pipes, particularly twin hot and cold-water pipes that unite in their discharge. ^Y^.—A pipe fitting for uniting two pipes at an angle of 45°. HISTORICAL INTRODUCTION. The very small degree of antiquity to which machine tools can lay claim appears forcibly in the sparse records of the state of the mechanic arts a century ago. [Illustration: GOURD, CAULDRON AND PIPKIN.] A few tools of a rude kind, such as trip-hammers (worked by water wheels), and a few special ones, which aimed at accuracy but were of limited application, such as “mills” for boring cannon, or “engines” for cutting the teeth of clock wheels, were almost their only representatives. The transmission of power was unthought of, except for the very limited distances which were possible with the ill-fitted “gudgeons” and “lanterns and trundles” of the old millwrights.