Photo of Big Ben in London, England

Weekly Gem #207 Timing is everything

Published 8/17/2019

Location: This weekly gem is located at the intersection of Westminster Bridge and the River Thames in London (see the Clue Me! Map).

When Big Ben was built in the mid 1800s, it was far and away the most massive gear-driven clock in the world. In a case of ‘fake it ‘til you make it,’ the clockmaker himself (a brazen chap named Dent) didn’t know how to make this clock work at the time he submitted a proposal to do so. Fortunately for him, construction delays are nothing new. The clock tower took almost 5 years longer than planned, and during that time, he was able to invent a way for his clock to keep time.

His solution was a Denison Double Three-Legged Gravity Escapement. Obvious now, but at the time? Needless to say, this isn’t your grandfather’s clock. But, consider a grandfather clock. It has a pendulum, which turns some gears at a precise pace. Those gears allow a weight to drop at an equally precise rate, and that weight is what turns the hands of the clock. Easy peasy. Of course, your grandfather clock is inside your house, and also inside a closed case. The pendulum and the clock face are enclosed because even a slight wind, or something miniscule resting on the hands, will reduce the accuracy of the clock, or even cause it to stop.

Dent had not one, but four clocks. Each exposed (but differently) to wind, snow, ice, rain, and the occasional perching bird. For example, a west wind presses in on the hands on the west face, blows across the north and south faces turning the hands into ‘wings,’ and the east face is unperturbed (although that is probably where the bird will perch).

Back to the Denison Double Three-Legged Gravity Escapement. Details aside, this device kept the pendulum swinging exactly right, regardless of what outside influences were doing to the hands. A stroke of genius.

Good enough, but here’s a riddle for you. When does a penny equal two fifths?

We’ll tell you in a minute. In the meantime, think about this. Three trains kept the clocks on time …

In the clock tower are three massive gear mechanisms (called trains), each made of dozens of cast metal parts which expand and contract with changes in weather. So, double three legs aside, the clocks were still prone to changing speed with the changing weather.

Dent determined that adding one penny to the pendulum raised the center of gravity ever so slightly, but enough to speed the clock by two fifths of a second per day. One penny equals two fifths.

Even today, Big Ben is tracked and monitored electronically, sensors scattered about. And, when the sensor shows the clock is running a bit off, the clockmaster goes down to the pendulum and places, or removes, one penny.

Time is up for this week. Check us out next week for a few interesting and little-known details about Big Ben.


Here's the hidden gem entry from our Clue Me! map.


Queen’s Timepiece


Big Ben

Why It's Interesting

All I have to say is, it’s definitely worth the time to visit

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