The Carpenterand

The Clock(A Tale of Quality)

In 1720 Sir Charles Pelham asked John Harrison to build a clock for his estate at Brocklesby Park in Northeast England.

Sir Charles Pelham John Harrison

1720

Harrison was not a clockmaker. He was a carpenter by trade. But he was a carpenter known for his obsession with quality.

More than two hundred years after it was first set in motion, the Brocklesby Park clock is still running.

Harrison would go on to build a series of clocks in an attempt to solve one of the great technological puzzles of his day; how to measure Longitude.

The solution would require a clock that was compact enough to be carried on a ship, could keep perfect time in stormy weather and in temperatures from Arctic ice to the noonday sun at the Equator. It would have to be able to shrug off variations in humidity and atmospheric pressure, and not be affected by local variations in the Earth’s magnetic field.

Harrison’s goal was to measure one abstract concept (Longitude) using another abstract concept (Time).

Clocks do not measure Time as such. We use the ticking of the clock as a proxy for the passage of time. We have also used – and sometimes still use – other proxies: grains of sand in an hourglass; water in a water clock; the burning of a candle; the filling in of a progress bar.

Similarly, there are no actual physical lines of longitude on our oceans and continents. They exist only in our minds and are sometimes shown on diagrams we call “maps”.

Prime Meridian, Greenwich, England

If you know the local time and the time at the Prime Meridian (in Greenwich, England) the difference between the two gives you your longitude. Time at the Prime Meridian is referred to as GMT (Greenwich Mean Time).

There are twenty-four hours in a day, 3600 in a circle. So a time difference of one hour equates to 360/24 degrees, or fifteen degrees. If your local time is later than GMT, you are East; if it is earlier than GMT, you are West.

Before he could attempt to measure Longitude, John Harrison had to master the art of measuring time.

When Harrison built his first clock, the United States had not yet been invented, Paris did not yet have an Eiffel Tower and medical procedures frequently involved leeches. Surgeons operated in their regular daytime clothes; anesthetics and sterile operating rooms lay far in the future.

Being a carpenter, Harrison used wood in much of his construction.

Critical components of the clock were made from dense woods such as Oak and Lignum Vitae (“Tree of Life”, also known as Guaiacum; indigenous to the Caribbean and South America).

Lignum Vitae

The teeth of his cog wheels were cut from the wood so that the grain ran out radially from the center of the cog. Each individual cog wheel was a subsystem of specially chosen and cut woods.

Lignum Vitae naturally secretes an oil, so the cog wheels are self-lubricating.

Clocks of Harrison’s time were prone to error as their pendulums could expand and contract with changes in ambient temperature.

Harrison developed a “gridiron” pendulum made up of alternating rods of different metal alloys; expansion in one metal was offset by expansion in the other and the pendulum’s behavior remained constant.

The clock included another self-correction mechanism. Harrison added a small hidden second pendulum inside the clock case.

As a clock ticks it may develop a wobble known as “Circular Error” which may make it run fast or slow. Harrison’s hidden second pendulum prevents Circular Error.

The Harrison clock included many other innovations, all designed to make the clock as accurate and maintenance-free as possible.

The clock is now in its 295th year; it has been running for approximately 2.5 million hours.

Does Harrison’s example have any relevance to software development?

Each part was tested as it was made, before being added to a component.

Each component was tested before being added to an assembly.

Each assembly was tested before becoming part of the clock.

Harrison built quality in at every step of construction. He unit tested individual parts, he tested integration between parts, he regression tested assemblies each time a new component was added. Harrison devoted more time to testing than to construction.

It would make no sense to try building software now that could still be running – and running well – 294 years from now.

Would a more modest horizon – ten years, fifteen years – make sense in the case of software?

There are well-tested standards and principles that address these questions. Many of them have acronyms we have encountered or will encounter in the near future.

Each of them reflects an ethos that John Harrison would have recognized and appreciated.

How would you build quality in as you go? How would you plan for ease of maintenance? How would you make it simple to replace parts without disrupting the entire system?

Postscript: Harrison did eventually crack the Longitude problem. After a few tries …

The Harrison “H4”

Chronometer

End