A balance wheel on a hairspring keeps excellent time, so long as nothing interferes with it. Something must interfere with it. The balance is also what meters out the mainspring's energy, and in every escapement of the eighteenth century the wheel driven by that spring stayed in contact with the balance through most of its swing — so the rate followed the state of the spring, the oil, the dirt and the weather. In a London workshop, Thomas Mudge put a pivoted lever between the two, and the balance was suddenly touched only briefly, twice a cycle, and left alone the rest of the time. The technical word is detached, and it is the most valuable property a timekeeper can have.

Imogen Hartley builds outward from there. A wound spring holds energy and gives it back unevenly, hard when tight and feeble when nearly spent; released freely it would empty in seconds. The escapement is the joint that holds it back and lets it out in equal instalments, while simultaneously pushing the oscillator to replace what friction removes. Those two duties work against one another, since every push disturbs the very thing whose steadiness is being relied upon.

The tick is that compromise made audible, and it decomposes into four events that repeat without variation. A tooth resting against a pallet while the balance swings free. The returning balance shoving the lever hard enough to drive the escape wheel briefly backwards and release the tooth. The freed tooth sliding down a sloped face, driving the lever and through it the balance. And the drop — a deliberate gap in which the tooth falls clear and nothing useful happens at all. That waste is not a fault. It is the clearance guaranteeing the parts never collide, and the book uses it as a case study in why machines meant to run for decades are built with slack in them.

There is a rival that was better and lost anyway. The detent escapement touches the oscillator even less and squanders almost nothing in friction, which is why marine chronometers adopted it. It also fails to restart itself if the balance stops, and it dislikes being knocked. A chronometer resting in gimbals can accept that; a watch on a wrist cannot. Robustness proved to be a performance characteristic, and the market judged it correctly.

Later chapters follow the patent record into the present: efforts to make the impulse independent of how wound the spring is, arrangements separating locking from impulse so that the oil-hungry sliding contact is reduced, paired and doubled escapements, components etched from silicon instead of cut from steel. The geometry has scarcely moved in two centuries. What changed is the material, the manufacture, and — given that an electronic oscillator surpasses a mechanical one on every measure the escapement exists to satisfy — the reason anyone still builds them at all.

Twenty-four purpose-drawn diagrams accompany the text, and twenty-five references, chiefly patents and horological sources, close it. Anyone seeking beat rates, amplitudes, lock and drop angles, efficiency percentages or the dates of Mudge's career will not find them here. Each is easy to obtain second-hand and impossible to verify that way, so each is named where it would have stood and then omitted.