MUJCA — how time got measured

A large ornate pocket watch movement displayed under glass in a museum case, dial removed to show the mechanism, controlled gallery lighting — Held flat it reads as a very large pocket watch. Turned over it is a machine with a fast balance and a remontoire.Photo: H4 timekeeper · Wikimedia Commons

I · Longitude · The lead entry

Harrison's H4

Finished in 1759 and now at the Royal Observatory, a large watch that kept time well enough to find longitude across an Atlantic crossing.

Three large machines were built on the assumption that a marine timekeeper had to resist the motion of a ship. The fourth abandoned the premise, and went to sea in a box.

Read the lead entry

Two to read next

Where the problem starts

An antique brass sextant on a chart table beside a chronometer in its box, window light from the left

I · Longitude

Why longitude is time

The Earth turns fifteen degrees an hour, so knowing the time at a reference place tells you how far around you are.

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Extreme close on an anchor escapement engaging an escape wheel, pallets and teeth sharp, everything else falling away

II · Escapement

What an escapement does

It releases the train one tooth at a time and returns a push to the oscillator, doing two jobs that fight each other.

Photo: Escapement of RiefNo549-1929 · Wikimedia Commons

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One mechanism, one error

The register · five of nineteen

Every clock on this site does one thing: it releases a wheel one tooth at a time, and gives something swinging a push to keep it going. What separates them is the error each mechanism was built to defeat.

A precision regulator clock movement, deadbeat escapement at the top

IDeadbeatGeorge GrahamDefeats Recoilthe wheel pushed backwards at every beat

A wooden clock movement with unusual pivoted arms

IIGrasshopperJohn HarrisonDefeats Frictionsliding contact, and oil that dries out

A watch movement with the balance and lever visible

IIILeverThomas MudgeDefeats Attachmenta balance still held by the train mid-swing

A gridiron pendulum of parallel brass and steel rods

IVGridironJohn HarrisonDefeats Heata pendulum that lengthens as the room warms

A tall cylindrical vacuum tank containing a pendulum

VFree pendulumW. H. ShorttDefeats Interferencethe impulse itself, and everything touching the rod

The whole register — all 19 entries

A pendulum bob and its suspension spring, dark background, single light — A bob and its suspension spring: the part of a clock that is asked to do nothing except swing at the same rate for ever.

III & IV · Compensation, then freedom

A pendulum lengthens as it warms, so the second gets longer with the room

Harrison answered with the gridiron: brass and steel rods alternating so that one metal’s expansion cancels the other’s. Later came mercury, and then an alloy that barely moves at all.

The free pendulum removed the last interference. A slave clock does the counting and the driving while the master swings in a vacuum tank, undisturbed — steady enough to show that the Earth’s own rotation is not.