page fifty

The Regulator

Forty-nine rooms, and every one of them was judged while you were still in it. Today's number sends a clockmaker to a Georgian house where a longcase clock has come by cart from the old place with its bob knocked askew. You have eight noons, a brass line on the south sill and the rating nut. Then you go home, and a month later the owner writes to say how it kept.

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Noon

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The sun's image takes about two minutes to cross the line; here time runs eight times fast. Mark when its leading edge touches the brass, and again when its trailing edge leaves it: the mean of the two is the clock's reading at apparent noon. Space or Enter marks too. The apprentice will do it for you, less carefully.

The rating nut
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The hands
The rating book

Rates are worked from one sight to the next, allowing for the hands if you set them. The column at 60°F takes off what the book says the warmth of the house did. A star means the nut was turned in between.

House
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The arrow keys walk to other days. Every day is a different house, owner, clock and month of weather.

What is real here and what is not

The pendulum, the equation of time, the expansion of the rods and the way a clock was rated are real. The houses, the owners and the clock's knocks are the day's number.

  1. 01

    Apparent and mean

    A clock keeps mean time; the sun keeps apparent time, and the two drift apart through the year by up to a quarter of an hour, because the earth's orbit is an ellipse and its axis is tilted. Clockmakers sold printed tables of the equation of time to go with their clocks. Here it is worked for the date, and the noon line tells you the clock's error only after you have applied it.

  2. 02

    The rating nut

    A seconds pendulum is about 39.1 inches long, and its period goes as the square root of its length, so shortening it by one part in a hundred thousand gains about 0.43 seconds a day. The nut under the bob screws it up or down; its thread and divisions set how much a division is worth. The book's figure is never quite the clock's, because the bob does not always follow the nut cleanly.

  3. 03

    Cold rods gain

    Steel and brass shrink in the cold, the pendulum shortens and the clock gains. Varnished deal hardly shrinks at all but swells with the damp. Harrison's gridiron and Graham's mercury jar were built to cancel the effect, and did so only as well as they were made. A clock regulated in a warm week will gain all winter unless it is left a little slow on purpose.

  4. 04

    What the page simplifies

    Each day's rate here comes from one mean temperature, the arc of the pendulum never changes, and the noon line is perfectly true to the meridian. Real makers also took equal altitudes of the sun with a quadrant, or used a transit instrument, and real clocks change rate as their oil thickens. The equation of time is a good modern approximation, not an eighteenth-century table.

Fifty rooms, and one number doing all the work. Today it knocks a bob askew, lets the house cool behind your back, and keeps the owner's letter until a month after you have gone.