Daina Bouquin: A war is on.
Daina Bouquin: And you are standing in the dark.
Daina Bouquin: It's 1943.
Daina Bouquin: You are on the roof of a building near Liverpool, wrapped in a heavy trench coat that's losing a fight against the damp, biting cold.
Daina Bouquin: The rim of a tin helmet presses hard against your forehead.
Daina Bouquin: In your numb hands you hold a bucket of water.
Daina Bouquin: You are looking up into the blackout, waiting for incendiary bombs to fall.
Daina Bouquin: You're up here because a Nazi propagandist on the radio has promised that by morning, this building, the Bidston Observatory, will be no more.
Daina Bouquin: Bombs do fall in the area.
Daina Bouquin: Windows shatter.
Daina Bouquin: But the observatory stands.
Daina Bouquin: And when your shift on fire watch ends, you go back downstairs, deep into the bunker-like basement to fight the Second World War with a pencil.
Daina Bouquin: Because you are a computer, and the Navy needs you.
Daina Bouquin: I'm Daina Bouquin, and this is Found in the Machine.
Daina Bouquin: Before the word computer meant a machine on a desk, it was a job title.
Daina Bouquin: Often the job title of a woman.
Daina Bouquin: In this case, the job title of six young women in a basement who are hired for their diligence, skill with numbers, and neat handwriting.
Daina Bouquin: On their desks are papers covered in columns of numbers, pages and pages of them.
Daina Bouquin: Hourly measurements of water levels taken at ports and harbors across Britain and beyond.
Daina Bouquin: Some of the data is decades old.
Daina Bouquin: Some arrived by courier this morning marked urgent.
Daina Bouquin: They are responsible for predicting the tides.
Daina Bouquin: They are working under a man named Arthur Doodson.
Daina Bouquin: Arthur is 53 years old, and he is a brilliant mathematician.
Daina Bouquin: He is also deaf.
Daina Bouquin: He cannot hear the German bombs exploding outside, and he cannot hear the metallic whirr of the machines in the other rooms.
Daina Bouquin: But in his own way, Arthur Doodson listens to the ocean better than anyone else on Earth.
Daina Bouquin: To Arthur, the tide is not chaos.
Daina Bouquin: It is a cord.
Daina Bouquin: A cord made of the moon and the sun and the shape of the ocean floor and the shoulder of every continent the water leans against.
Daina Bouquin: Pull those notes apart and you have the variables that make the tide.
Daina Bouquin: Scientists realized this in the nineteenth century.
Daina Bouquin: They realized that if you could measure each of the influences that shape the water, you could predict what the ocean would do next.
Daina Bouquin: But you needed the celestial pulls, the local quirks of the coastline, the depth.
Daina Bouquin: You needed these harmonic constituents.
Daina Bouquin: Even if you had those measurements though, you couldn't do this work quickly, and not in your head.
Daina Bouquin: So they built mechanical marvels.
Daina Bouquin: The Bidston Observatory has two of them.
Daina Bouquin: One was the Bidston-Kelvin machine, a descendant of Lord Kelvin's original tide predictor design installed for Doodson in the 1920s.
Daina Bouquin: The other was the Roberts-Légé machine, built in 1906 and later acquired for Bidston.
Daina Bouquin: It weighs over a ton.
Daina Bouquin: They are analog computers.
Daina Bouquin: Glorious, intricate contrivances of shining brass and steel filled with wheels and gears and pulleys.
Daina Bouquin: The way they work is mesmerizing.
Daina Bouquin: There is a wire winding its way over and under dozens of pulley wheels.
Daina Bouquin: Each wheel represents a different force.
Daina Bouquin: The pull of the moon, the pull of the sun, the wobble of an orbit.
Daina Bouquin: The women set the dials and turn a crank to run the ocean forward in time.
Daina Bouquin: It takes four hours to draw a year, but the setup, the work to tell the machine where to start, takes days.
Daina Bouquin: So day after day the women look at pages of hourly water levels.
Daina Bouquin: They lay cardboard stencils with cutout holes over the data, multiplying the numbers that show through by specific weights.
Daina Bouquin: They do the differencing.
Daina Bouquin: They do the smoothing.
Daina Bouquin: Down in the basement they find the harmonic constants.
Daina Bouquin: Then they set the brass dials and crank the wheel.
Daina Bouquin: The wire pulls tight, moving a pen across a rotating paper drum, drawing a rolling line.
Daina Bouquin: The shape of a future ocean, and the time it will take to rise.
Daina Bouquin: Across the English Channel, German Field Marshal Erwin Rommel paces the sands of France.
Daina Bouquin: He looks out at the water.
Daina Bouquin: The tides are massive.
Daina Bouquin: The vertical distance from low to high tide exceeds six meters, nearly twenty feet.
Daina Bouquin: When the tide comes in, it rushes at terrifying speed.
Daina Bouquin: The water rises roughly a foot every 15 minutes.
Daina Bouquin: Rommel tells his officers the Allies will come at high water.
Daina Bouquin: He cannot fathom that they would force soldiers to run across hundreds of yards of open, muddy beach under heavy machine gun fire.
Daina Bouquin: He prepares for a high tide invasion.
Daina Bouquin: He orders the beaches sewn with iron.
Daina Bouquin: Thousands of obstacles and mines, welded steel crossbars taller than a man, wooden stakes angled at the sea, tipped with more mines and captured artillery shells.
Daina Bouquin: Three-ton Belgian gates rolled into the shallows.
Daina Bouquin: He places them exactly where they will be hidden just beneath the surface as the dark water rises to midtide, ready to rip the bottoms out of Allied landing craft.
Daina Bouquin: To detonate when the ships come near.
Daina Bouquin: But Allied reconnaissance planes photograph the traps.
Daina Bouquin: The commanders realize that a high tide landing is a death trap.
Daina Bouquin: They realize they have to land at low tide, so the demolition engineers can see the steel obstacles and blow them to pieces.
Daina Bouquin: But the tide must also be rising, rising fast so the boats can unload their men and float back out without getting stranded on the sand.
Daina Bouquin: And they also need a bright, late rising moon.
Daina Bouquin: Otherwise the paratroopers won't be able to see their drop zones.
Daina Bouquin: And the daylight.
Daina Bouquin: They need an hour of daylight to bombard the coast before the troops hit the sand.
Daina Bouquin: Daylight has to coincide with low tide.
Daina Bouquin: And the invasion is huge.
Daina Bouquin: It spans five beaches Utah, Omaha, Gold, Juno, and Sword.
Daina Bouquin: The water sloshes through the English Channel like a funnel.
Daina Bouquin: The exact minute of low tide differs by more than an hour from one end of the invasion to the other.
Daina Bouquin: The landings have to be meticulously staggered.
Daina Bouquin: They need to know exactly what the water will do.
Daina Bouquin: They need someone to tell them when the earth will cooperate.
Daina Bouquin: They need the moon, the sun, the dawn, and the tides to align.
Daina Bouquin: On October 9th, 1943, Arthur Doodson receives a three-page handwritten letter marked MOST URGENT.
Daina Bouquin: It is from Commander William Ian Farquarson, the Admiralty's superintendent of tides.
Daina Bouquin: Farquarson cannot tell Doodson the biggest secret of the war.
Daina Bouquin: He cannot say "Normandy." Instead, he provides what data he can for a location he calls "Position Z." He writes quote, "The place is nameless and the constants inferred.
Daina Bouquin: There is in fact very little data for it.
Daina Bouquin: I am gambling on the inferred shallow water constants giving something like the right answer." Doodson looks at the numbers.
Daina Bouquin: Constants inferred from sparse shallow water readings of nearby ports gathered by British reconnaissance teams in small boats and submarines in the night.
Daina Bouquin: Arthur knows the tides and guesses the location of Position Z, but he says nothing.
Daina Bouquin: He and the six women just get to work.
Daina Bouquin: They do the painstaking arithmetic, the harmonic analysis.
Daina Bouquin: They calibrate the brass wheels.
Daina Bouquin: They turn the crank.
Daina Bouquin: The pulleys rise and fall.
Daina Bouquin: They carefully translate the machine's raw output into tide tables.
Daina Bouquin: They map the future of a nameless coast.
Daina Bouquin: There are only three days in June 1944 that work.
Daina Bouquin: The fifth, the sixth, and the seventh. General Dwight D.
Daina Bouquin: Eisenhower chooses June 5th for the invasion, but a massive storm forces a delay.
Daina Bouquin: It doesn't matter what the tides do if the air won't cooperate.
Daina Bouquin: Rommel sees the same rough weather.
Daina Bouquin: He looks at the tides and concludes they are completely wrong for an invasion.
Daina Bouquin: Believing the Allies won't come, he leaves his headquarters and drives to Germany to celebrate his wife's birthday.
Daina Bouquin: But the Allies do come.
Daina Bouquin: On the morning of June 6, 1944, the largest amphibious invasion in the history of the world arrives on the coast of Normandy.
Daina Bouquin: The tide predictions hold.
Daina Bouquin: Water comes rushing in, rising and rising.
Daina Bouquin: On Omaha Beach, the American demolition teams have only minutes to blast their channels before the ocean swallows the traps.
Daina Bouquin: It is horrific.
Daina Bouquin: They knew it would be.
Daina Bouquin: Men in wool uniforms wading through cold water, weighed down by their packs, running toward the cliffs.
Daina Bouquin: Toward the wire.
Daina Bouquin: Toward the guns on the bluffs.
Daina Bouquin: Machine gun fire from the heights.
Daina Bouquin: The blast of heavy artillery, constant and deafening.
Daina Bouquin: Men waiting for their turn to run, sitting in small boats, breathing in engine fumes, watching their friends fall.
Daina Bouquin: More than half the engineers on Omaha are killed.
Daina Bouquin: And the fast rising water lifts the landing craft exactly as planned.
Daina Bouquin: They drop their ramps, they unload the troops, they reverse back into the channel.
Daina Bouquin: The tide does what they said it would do.
Daina Bouquin: The liberation of Europe was paid for in blood and secured by the unimaginable bravery of the men who plunged into the surf and ran into a killing field.
Daina Bouquin: But beneath the battle, beneath the smoke and the cacophony of the guns, there was a rhythm, a quieter one, dictated by the moon and the sun and the shape of the ground the men ran across.
Daina Bouquin: It was drawn in pencil in a basement near Liverpool by a deaf mathematician and six young women whose names were left off the public records.
Daina Bouquin: But they did have names.
Daina Bouquin: And if you look hard, if you look really hard, you can find two of them: Joan Rossiter and Estelle Gilbert.
Daina Bouquin: Their names are mentioned in reference nine of a Physics Today article published in 2011, sixty-seven years after D-Day.
Daina Bouquin: The author had spoken to them a decade earlier and wrote down their names.
Daina Bouquin: The other four are unknown.
Daina Bouquin: We only know the names of some of the women who came later, and we only know them because they wrote the records themselves.
Daina Bouquin: They wrote down what it was like to operate the machines.
Daina Bouquin: They smiled in a group photo taken sometime after the war.
Daina Bouquin: Joan Rossiter and Estelle Gilbert are not in the photo.
Daina Bouquin: The women who helped calculate the tides in 1944 were swallowed by a job title.
Daina Bouquin: They were just Computers.
Daina Bouquin: They were diligent and good with numbers.
Daina Bouquin: They had neat handwriting.
Daina Bouquin: And when the tide came in on the morning of June 6th, 1944, they were already working on the next tide.
Daina Bouquin: And the one after that.
Daina Bouquin: Because a war was on.
Daina Bouquin: And the Navy needed them.
Daina Bouquin: I'm Daina Bouquin, and this is Found in the Machine.
Daina Bouquin: If you want to support this show, leave a rating or a review wherever you listen.
Daina Bouquin: Or share this episode with a friend.
Daina Bouquin: And if you'd like to hear what didn't make it into this episode, or see that group photo of the women who calculated the tides after the war, go to notes.foundinthemachine.com.
Daina Bouquin: Thanks for listening.