The Detent Escapement: The Chronometer's Heartbeat, and a Surprise in Newport

In 1997 a pocket watch came into the collection of the Newport Historical Society, donated by descendants of a Newport merchant named Peleg Clarke. It had stayed in the family for more than two centuries, and for most of that time it was simply that: a fine old pocket watch. Nearly twenty years later, a staff member carried it across the Atlantic to the Royal Observatory at Greenwich, where curator of horology Rory McEvoy confirmed something far more interesting. The "pocket watch" was an early precision marine timekeeper by the London maker John Arnold, made around 1772, and the fourth in a short series of experimental longitude watches. Of that series, only one other example, Arnold's number three in the British Museum, is known to survive.

What made the Newport watch special was hidden behind the dial: a pivoted detent escapement. To understand why that matters, and why a version of the same idea went on to govern ships' chronometers for well over a century, we need to look at one of the most elegant, and most temperamental, mechanisms in all of horology.

What an escapement has to do

Every mechanical timepiece has the same basic problem. A mainspring wants to unwind all at once; a balance wheel wants to swing back and forth at a steady rate. The escapement sits between them. It lets the gear train advance in small, equal steps, and on each step it hands a little energy to the balance to keep it swinging.

Every time the escapement touches the balance, it disturbs it: friction, aging oil and tiny variations in the push all creep into the rate. The ideal escapement would leave the balance completely alone for as much of its swing as possible, touch it only briefly, and do so in exactly the same way every time.

The detent escapement comes remarkably close to that ideal. Horologists call it a "free" or "detached" escapement: for most of each oscillation the balance swings with no contact at all with the rest of the movement.

How the detent works

Picture four main players: an escape wheel with pointed teeth, a slender arm called the detent, and two small pallets mounted on a roller on the balance staff.

The detent carries a jewel, the locking stone, which normally rests against a tooth of the escape wheel and holds the whole train still. The balance, meanwhile, swings freely.

As the balance swings in one direction, a small discharging pallet on its roller nudges a very fine spring attached to the detent. This flexes the detent just enough to pull the locking stone out of the way. The escape wheel is released, and a tooth immediately catches up with a second, larger pallet on the roller, the impulse pallet, and gives the balance a single sharp push. Almost at once the detent springs back, the locking stone catches the next tooth, and the train is locked again. The balance carries on, untouched, to the end of its swing.

On the return swing something clever happens. The discharging pallet passes the fine spring again, but from the other side. This time the spring, often called the passing spring and traditionally made of gold, simply bends aside and lets the pallet slip past without moving the detent at all. Nothing is unlocked, no impulse is given, and the balance completes its swing in silence.

So the detent is a single-beat escapement: the balance receives one impulse for every two swings, rather than one on every swing as in the familiar lever escapement found in most mechanical watches. There is also very little sliding between the escape wheel tooth and the pallets, which means the escapement needs little or no oil at the point of impulse. Since deteriorating lubricant was one of the great enemies of long-term accuracy, that was a major advantage.

Two forms: pivoted and spring

The idea came in more than one shape. The French maker Pierre Le Roy is generally credited with an early detent-type escapement around 1748, but it was in England, in the decades that followed, that the design was turned into something practical for the sea.

In the pivoted detent, the detent is a small lever turning on its own pivots, returned to its resting position by a separate fine spring. This is the type found in the Newport watch, and Arnold's version was regarded as a real step forward.

In the spring detent, the pivots are done away with altogether. The detent is mounted on a thin blade of steel that is itself the spring: it flexes to unlock and springs back on its own. No pivots means no pivot friction and nothing there to oil. This is the form most closely associated with Thomas Earnshaw, and it became the form that marine chronometers used almost universally.

A bitter quarrel over credit

Earnshaw, born in Ashton-under-Lyne, Lancashire, in 1749, was trained as a watchmaker in London and by about 1780 was skilled in escapement work and jewelling. He patented his spring detent through another watchmaker, Thomas Wright, who acted as sponsor, and from 1783 other makers could use the design for a fee of one guinea per watch, provided it was marked "Wright's Patent."

The difficulty was that Arnold had also patented a form of spring detent in 1782. Earnshaw had shown his design to friends of Arnold, and he came to believe his idea had been borrowed. The priority dispute that followed dragged on for years and drew in the Board of Longitude, the government body that judged and rewarded advances in finding longitude at sea.

In March 1804 the Board took the unusual step of ordering both Earnshaw and Arnold's son, John Roger Arnold, to build large demonstration models of their escapements so it could see exactly how each worked. Earnshaw's model survives at Royal Museums Greenwich, rather roughly assembled, and it cost the Board considerably more than the Arnold model did. In the end, according to the museum, both men were each awarded £3,000 for their contributions. Who truly deserves credit for the spring detent is still debated today.

Why it ruled the seas, and stayed off the wrist

Whatever the quarrel, the result was a mechanism that combined high accuracy with something just as important: it could be made in quantity. Earnshaw built his chronometers to a standard plan, which allowed specialist outworkers to produce movements in batches. That brought the price of a reliable ship's timekeeper within reach of ordinary navies and merchant fleets, rather than a handful of famous expeditions. An Earnshaw spring detent chronometer was among the timekeepers carried aboard HMS Beagle when it sailed in 1831 with the young Charles Darwin, and it is now in the British Museum.

The design proved so sound that marine chronometers kept using it, with little fundamental change, until quartz timekeeping took over in the twentieth century.

Yet you will almost never find one in a wristwatch, and the reasons are instructive. The detent is easily "tripped": a sudden knock can jolt the locking stone free at the wrong moment, letting the escape wheel run and upsetting the rate or stopping the watch altogether. The safety devices that protect a lever escapement from such shocks are hard to adapt to it. And a detent escapement is often not self-starting; if the balance stops, it may need a gentle shake to get going again. On a ship, a chronometer sat in a box, slung in gimbals and protected from shock, so none of this mattered. On a human arm, it matters a great deal. Only a handful of modern independent watchmakers have attempted wristwatches with a detent, and they have had to invent their own safeguards to do it.

A Newport connection to a London rivalry

All of which brings us back to Peleg Clarke's watch. He acquired it in 1792, probably during a trip to London and perhaps from Arnold himself. Clarke was a prosperous Newport merchant, descended from one of the town's early English settlers, and he had been an eyewitness to the Boston Tea Party; a letter he wrote describing it is also held by the Newport Historical Society.

His timepiece is a survivor from the very years when Arnold, Earnshaw and others were racing to make longitude-finding practical, small enough to carry in a pocket and built around an escapement that was then at the frontier of precision. It is a reminder that important pieces of horological history sometimes sit quietly in a local collection, waiting for someone to look more closely at what is ticking inside.