Glossary

The mass of masonry at each end of an arch that takes its outward push.

On the plates: Arch and Vault

Said of a lens of two glasses, crown and flint, paired so that two colours focus at the same point, which greatly reduces coloured fringes.

On the plates: Microscope, Telescope

Jelly made from red seaweed; mixed with nutrients and set in a dish, it gives bacteria and moulds a surface to grow on.

On the plates: Penicillin

Holding finished glass or metal at a set heat, then cooling it slowly so that no stresses stay locked inside.

On the plates: Steel, Glass

Simple plough with a pointed share and no mouldboard; it scratches a furrow and breaks the soil without turning it over.

On the plates: Plough

Part moved by or moving through a magnetic field: the wound rotor of a dynamo, or the iron bar an electromagnet pulls in a telegraph.

On the plates: Electric Generator, Electric Motor, Electric Telegraph

Sealed vessel in which steam is heated above 100 °C under pressure, to sterilise instruments or to cure materials such as rubber.

On the plates: Hygiene & Antisepsis, Canning & Pasteurisation

The lower millstone, fixed in place; the grain is ground between it and the runner stone turning above.

On the plates: Windmill, Water Wheel

Tall shaft furnace, blown with forced air, hot enough to melt iron; the metal takes up carbon and is tapped as liquid cast iron.

On the plates: Iron Smelting

Two sets of pulleys joined by one rope; the more strands carry the load, the less force is needed and the more rope is pulled.

On the plates: Crane & Pulley

Spongy lump of iron and slag taken hot from a bloomery, then hammered to squeeze out the slag and consolidate the iron.

On the plates: Iron Smelting

Small clay furnace in which charcoal turns iron ore into solid iron without melting it, leaving a spongy lump called a bloom.

On the plates: Iron Smelting

Iron tube, over a metre long, used to gather molten glass on one end and inflate it by blowing through the other.

On the plates: Glass

Shaped lobe on a turning shaft that pushes a rod or lever at a set moment in each turn, then lets it fall back.

On the plates: Internal Combustion Engine, Paper, Sewing Machine

Dark room or box in which light entering through a small hole or lens throws an upside-down image of the scene outside.

On the plates: Photography

Telephone microphone in which the voice presses a diaphragm on carbon granules, changing their resistance and so the current on the line.

On the plates: Telephone

Slow reaction in which slaked lime takes carbon dioxide from the air and turns back into limestone, so lime mortar hardens over months.

On the plates: Reinforced Concrete, Portland Cement, Lime and lime mortar

Iron with about 2 to 4% carbon, which melts at a lower heat and is poured into moulds; hard but brittle.

On the plates: Iron Smelting, Plough, Steel, Internal Combustion Engine

Stream of electrons that leaves the negative electrode of a tube emptied of air; where it strikes glass or metal it gives light or X-rays.

On the plates: X-Ray Imaging

Temporary timber frame that carries the voussoirs of an arch or vault until the keystone is in.

On the plates: Arch and Vault

Early radio detector: a glass tube of loose metal filings that cling together and conduct when a radio wave arrives.

On the plates: Radio

Split ring on the shaft of a dynamo that reverses the connections every half turn, so the current it sends out flows one way only.

On the plates: Electric Generator, Electric Motor

Small adjustable tray held in one hand, in which the compositor lines up type letter by letter to the width of the line.

On the plates: Printing Press

Smooth, shell-shaped break of flint, obsidian or glass, which makes thin sharp edges possible.

On the plates: Stone Tools

Rod jointed at both ends that links a piston or treadle to a crank, turning a push and pull into rotation.

On the plates: Steam Engine, Internal Combustion Engine, Steam Locomotive

Vertical blade fixed in the beam of a plough, in front of the share, that cuts the soil on the land side.

On the plates: Plough

Mild disease of cattle caused by a virus related to smallpox; Jenner showed in 1796 that catching it protects people against smallpox.

On the plates: Vaccination

Shaft with offset cranks that turns the back-and-forth movement of the pistons, passed on by connecting rods, into rotation.

On the plates: Internal Combustion Engine

Heat-proof pot, usually of fired clay or clay and graphite, in which metal or glass is melted.

On the plates: Steel, Incandescent Light Bulb, Bronze, Charcoal

Old unit of length, from the elbow to the tip of the middle finger; surviving Egyptian royal cubit rods measure about 52 to 53 cm.

On the plates: Measures and standards

Soft solid that forms when milk, or rubber latex, coagulates and separates from its watery liquid.

On the plates: Cheese, Vulcanised Rubber

Photograph made directly on a silver-coated copper plate, announced in 1839; each image is unique, since there is no negative to copy.

On the plates: Photography

Angle between true north and the direction a compass needle points; it differs from place to place and drifts over the years.

On the plates: Magnetic Compass, Celestial Navigation

Unit of a lens's power: one divided by its focal length in metres, so a 0.5 m lens is 2 dioptres.

On the plates: Telescope, Eyeglasses

Staff that holds the unspun flax or wool, from which the spinner draws a few fibres at a time.

On the plates: Spinning Wheel

Adding a tiny, controlled amount of another element to a semiconductor crystal to change how well it conducts.

On the plates: Transistor, Solar cell

Part of a clock that lets the gear train advance one tooth at a time and gives each swing of the regulator a small push.

On the plates: Mechanical Clock

Large wooden mill wheel with pegs, the cogs, set into one face of its rim; it drives a lantern pinion at a right angle.

On the plates: Windmill, Gears, Water Wheel

Small wind wheel at the back of a windmill's cap, set across the sails, which turns the cap until the sails face the wind.

On the plates: Windmill, Wind Turbine

Small toothed bar under the needle plate that rises, moves the fabric forward one stitch length, then drops back.

On the plates: Sewing Machine

Heavy wheel on a shaft whose momentum carries the machine through the moments when nothing drives it, and smooths its turning.

On the plates: Steam Engine, Internal Combustion Engine, Potter's Wheel, Sewing Machine

Distance from a lens or curved mirror to its focus, where parallel rays meet or, for a diverging lens, seem to come from.

On the plates: Microscope, Telescope, Eyeglasses

Horizontal bar with small weights near its ends, rocked to and fro by the verge; moving the weights made an early clock faster or slower.

On the plates: Mechanical Clock

Mould of boards, or of facing stones left in place, that holds wet concrete in shape until it has set.

On the plates: Reinforced Concrete, Portland Cement, Arch and Vault, Roman Concrete

Engine cycle in four piston strokes, over two turns of the crankshaft: intake, compression, power and exhaust.

On the plates: Internal Combustion Engine

Lead sulphide, a grey crystal; touched by a fine wire, it lets current through in one direction only and so detects radio signals.

On the plates: Transistor, Radio

Thin coat of glass fused onto pottery in the kiln, which seals the pores and can add colour and shine.

On the plates: Pottery, Distillation, Glass

Rod or blade of a sundial whose shadow marks the hour; on most dials its edge points at the celestial pole.

On the plates: Sundial

Large stone tool, worked on both faces into a pointed or oval shape, made for well over a million years.

On the plates: Control of Fire, Stone Tools

A loop of cord round one warp thread, tied to a rod; lifting the rod lifts every thread so tied and opens the shed.

On the plates: Weaving

Two coils on an iron core with an automatic switch, which turn the low voltage of a battery into pulses of thousands of volts.

On the plates: Transformer, Radio, X-Ray Imaging

Closed pipe that carries water down one side of a valley and up the other under its own pressure, ending slightly lower than it began.

On the plates: Aqueduct

The voussoir at the top of an arch, set last; once it is in, the arch holds itself.

On the plates: Arch and Vault

Oven built to fire pottery, bricks or lime, or to dry malt, at a heat the worker can raise and hold.

On the plates: Writing, Portland Cement, Weaving, Pottery, Lime and lime mortar, Bricks, Bronze, Charcoal, Roman Concrete, Beer

Shaping flint or similar stone by striking or pressing off flakes; the knapper works flake by flake until the tool has its edge.

On the plates: Stone Tools

Small gear made of round wooden rods held between two discs, like a cage, which the pegs of a larger wheel push round.

On the plates: Gears, Mechanical Clock

Triangular sail hung from a long yard slanted across the mast, which lets a ship sail closer to the wind.

On the plates: Sail

The path the compressive force follows through an arch; the arch stands while this line stays within its thickness.

On the plates: Arch and Vault

Machine stitch in which an upper and a lower thread interlock inside the fabric; unlike a chain stitch, it does not unravel when pulled.

On the plates: Sewing Machine

Piece of magnetite that is naturally magnetised; it attracts iron and can magnetise a needle stroked across it.

On the plates: Magnetic Compass

Casting from a wax model coated in clay or plaster; heat melts the wax out and molten metal fills the space it leaves.

On the plates: Bronze

Strongly alkaline solution, once leached from wood ash and today made from caustic soda or potash; it burns skin and eyes.

On the plates: Paper, Leather Tanning, Soap, Dyeing

Grain, usually barley, soaked until it sprouts, then dried in a kiln; its enzymes later turn the starch into sugar.

On the plates: Beer

Metal salt, most often alum, applied to a fibre before or with the dye so that the colour binds to it and resists washing.

On the plates: Dyeing

Curved board of a plough, behind the share, that lifts the cut slice of soil and turns it over.

On the plates: Plough

Separate characters, of baked clay, wood or cast metal, set into lines and pages, printed, then taken apart and used again.

On the plates: Printing Press

Natural soda salt that crusts dry lake beds in Egypt; the ancient glassmakers' main flux, which lets sand melt at a lower heat.

On the plates: Glass

Water wheel fed at the top: water fills buckets on one side and their weight turns it, using most of the fall's energy.

On the plates: Water Turbine, Water Wheel

The smaller gear of a pair; driven by a large wheel it turns faster, and driving one it gives more force.

On the plates: Gears, Mechanical Clock

Pointed or cutting blade at the foot of a plough that opens the soil or slices it flat beneath the furrow.

On the plates: Iron Smelting

Volcanic ash, named after Pozzuoli near Naples, that reacts with lime and water to make a mortar able to set under water.

On the plates: Portland Cement, Roman Concrete

Loss, with age, of the eye's power to focus on near things, because its lens grows stiffer; reading glasses correct it.

On the plates: Eyeglasses

Steel bar with a reversed letter cut in relief on its end, struck into copper to make the matrix in which that letter is cast.

On the plates: Printing Press, Steel, Aluminium

Calcium oxide, made by burning limestone in a kiln; it reacts with water so strongly that it heats up and can boil it.

On the plates: Portland Cement, Lime and lime mortar, Leather Tanning, Roman Concrete

Fluid that boils at a low temperature, taking in heat as it evaporates; once compressed, it gives that heat out as it condenses.

On the plates: Refrigeration

Enzyme preparation, traditionally taken from a young calf's stomach, that makes milk set into curd.

On the plates: Cheese

Long straight alley or shed where yarns are stretched out and twisted together into rope.

On the plates: Rope & Cordage

The upper millstone, which turns on its spindle above the fixed bed stone and grinds the grain between them.

On the plates: Windmill, Water Wheel

Potassium nitrate, a salt that crystallises where organic waste rots in soil or on walls; once used to cure meat.

On the plates: Salting & Curing, Gunpowder

Reaction of a fat or oil with a strong alkali such as lye, which gives soap and glycerol.

On the plates: Soap

The gap opened between raised and lowered warp threads, through which the weft is passed.

On the plates: Weaving

Boat-shaped holder for a spool of thread; in a sewing machine it carries the lower thread through the loop of the upper one.

On the plates: Sewing Machine

Glassy waste of smelting or melting metal: the ore's stone and impurities, with fuel ash and some metal, drawn off or skimmed away.

On the plates: Iron Smelting, Portland Cement

Calcium hydroxide, made by adding water to quicklime; kept as a soft putty or dry powder, it is the binder of lime mortar.

On the plates: Portland Cement, Lime and lime mortar, Leather Tanning, Roman Concrete, Dyeing

Narrow air gap between two metal knobs across which a high voltage jumps as a spark; early transmitters made their radio waves this way.

On the plates: Radio

Tiny cell that a mould or certain bacteria form to spread or to wait out bad conditions; bacterial spores survive boiling, drying and many disinfectants.

On the plates: Hygiene & Antisepsis, Penicillin, Crop Rotation, Canning & Pasteurisation, Leather Tanning, Salting & Curing

The lowest voussoir of an arch, resting on its support where the curve begins.

On the plates: Arch and Vault

Four-sided sail hung from a yard across the ship; it works best with the wind from behind.

On the plates: Sail

Plant compound, drawn from bark, wood, leaves or galls, that binds to the collagen of a hide and turns it into leather that does not rot.

On the plates: Leather Tanning

Sand, crushed shell, old potsherds or straw mixed into clay so that it dries and fires with less shrinking and cracking.

On the plates: Steel, Pottery, Bricks

Large wheel turned by people walking inside it, which winds a rope on its axle to lift heavy loads with a crane.

On the plates: Crane & Pulley

Clay or metal pipe that carries the air from the bellows into the fire of a furnace or forge.

On the plates: Iron Smelting

Water wheel whose lower paddles dip into a stream and are pushed by the current; simple to build but less efficient.

On the plates: Water Turbine, Water Wheel

Sealed glass tube emptied of air; with a heated cathode and other electrodes inside, it can rectify or amplify a current, as in early radios.

On the plates: Computer, Transistor, Incandescent Light Bulb, Radio, X-Ray Imaging

Deliberate infection with matter from a smallpox sore, to cause a milder illness and lasting protection; some died of it, and it could spread smallpox.

On the plates: Vaccination

One of the wedge-shaped stones or bricks that together make up an arch.

On the plates: Arch and Vault

Heating raw rubber with sulphur so that its long molecules link together; it keeps its shape and stretch over a wider range of temperatures.

On the plates: Vulcanised Rubber

Fixed obstacles inside a lock; only a key whose bit has notches in the matching places can turn past them.

On the plates: Hygiene & Antisepsis, Lock & Key

The threads stretched lengthwise on a loom and held taut; the weft is passed through them.

On the plates: Weaving

The thread passed across the warp, over one thread and under the next, to make the cloth.

On the plates: Weaving

Small weight of stone, clay or wood fitted on a hand spindle so that it keeps turning longer.

On the plates: Weaving, Spinning Wheel

The Wright brothers' way of rolling an aircraft: wires twisted the two wingtips in opposite directions so that one rose and the other dropped.

On the plates: Aeroplane

Plant of the cabbage family (Isatis tinctoria) whose leaves give the same blue as indigo, in smaller amounts; the blue of European dyers before imported indigo took its place.

On the plates: Dyeing

Sweet liquid drained from malt soaked in hot water; boiled with hops and cooled, it is fermented by yeast into beer.

On the plates: Beer

Iron with very little carbon, worked under the hammer; tough and easy to forge and weld, with threads of slag inside.

On the plates: Iron Smelting, Steel, Mechanical Clock