=Gear.= A term applied to a piece of mechanism that accomplishes a single object: thus a valve-gear operates a valve; a steering-gear steers the vessel.

=Geared.= Placed in gear or connected together.

=Gear-wheel.= A wheel provided with teeth to engage with similar teeth upon another wheel.

=Generating-circle.= The circle that is supposed to move in the construction of cycloidal curves.

=Gib.= 1. A piece that may be set up to take up the wear. 2. A piece for holding a strap, and forming an ab.u.t.ting piece for a key.

=Gimlet.= A wood-boring hand tool, having a threaded conical end to pull it to its cut.

=Gimlet-bit.= A fluted gimlet having no thread at its end, but a spiral flute so shaped as to pull the bit forward to its cut.

=Gland.= A piece enveloping a stem and used to make a tight working joint.

=Globe-valve.= A valve, having a spherical body, used in pipe-work.

=Goose-neck.= A frame affording a fulcrum for a ratchet brace.

=Gouge.= A wood-cutting hand tool that is curved in its cross-section.

=Gravis.= A hand tool, rectangular in cross-section and having cutting edges at its end that are formed by grinding the end face at an acute angle to the body of the tool.

=Groove-cam.= A cam in which the actuating surface is in the form of a groove.

=Ground joint.= A joint that is finished by grinding the parts together, usually with oil and emery.

=Guide-bar= _or_ =slide-bar.= A bar that forms a guide for the crosshead of an engine or other moving piece.

=Gum.= 1. The bottom of the s.p.a.ce between saw teeth. 2. A rubber-like substance formed of oil that has dried.

H

=Hack.= A tool used for cutting iron in two under a steam hammer.

=Hack-saw.= A saw held in a frame and used by hand for cutting metal.

=Half-check joint.= A joint in which a piece is let into the other, so that the surfaces come level.

=Half-round file.= A file that is half round in cross-section.

=Hand-file.= A superior cla.s.s of file that is parallel in width and thickest in the middle of its length.

=Hand-hammer.= A hammer that can be used in one hand.

=Hand-lathe.= A lathe with which hand cutting tools are used.

=Hand-nut.= A nut that may be screwed up by hand without the aid of a wrench.

=Hand-planer.= A wood-planing machine in which the work is fed by hand.

=Hand-rest.= The rest on which hand-turning tools are supported in a lathe.

=Hand-tap.= A tap that is used by hand.

=Hand-vise.= A small vise for use in the hand.

=Hanger.= A frame that is bolted to another frame or part, and carries another piece, usually a shaft of some kind.

=Hardened.= Hardened steel is that which has been made hard by heating to a cherry red and suddenly cooling it, usually by quenching it in water.

=Hardy.= A blacksmith's chisel that fits into the anvil.

=Heading-block.= A block used in upsetting the heads of bolts or pins.

=Heart-shake.= A split radiating from the centre of a log.

=Heating-surface.= That part of the surface of a steam-boiler that receives heat on one side and has water on the other.

=Heel-tool.= A hand turning tool having a projecting heel to cross the tool rest, and usually held in a wooden stock or handle.

=Herring-bone tooth.= A form of gear wheel tooth in which the tooth, instead of pa.s.sing direct across the wheel face, curves partly around the circ.u.mference and then back again, so that the two ends of the tooth only are opposite to each other.

=Hindley's-screw.= A short length of screw used to drive a worm wheel, and sometimes termed an endless screw.

=Hob= _or_ =hub.= A tool for cutting the threads on screw cutting tools, such as chaser dies.

=Hour-gla.s.s screw.= A worm or tangent screw which is formed to envelop part of the arc of circ.u.mference of a worm wheel, and therefore a.s.sumes in outline the form of a sand hour-gla.s.s.

=Hunting-tooth.= An extra tooth put into a pair of gear wheels that would otherwise contain the same number of teeth, the object being to prevent the same teeth from always falling together.

=Hypocycloid= (h[=i]-po-s[=i]'kloid). A cycloidal curve in which the rolling circle is rolled within the fixed or base circle.

I

=Idle pulley= _or_ =guide pulley.= A pulley employed to guide a belt.

=Independent chuck.= A chuck in which each jaw is operated separately.

=Index-plate.= A plate having holes or notches accurately dividing a circle into equal divisions.

=Inserted-tooth cutter.= A cutter in which the teeth are inserted in a disc or head.

=Inside calipers.= Calipers used to measure inside dimensions, as boxes, recesses, etc.

=Intermediates.= The wheels that are between the front driver and last follower of a train of gear wheels.