WEBVTT 00:00:19.519 --> 00:00:21.039 A war is on. 00:00:21.039 --> 00:00:24.640 And you are standing in the dark. 00:00:24.640 --> 00:00:27.280 It's 1943. 00:00:27.280 --> 00:00:37.039 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. 00:00:37.039 --> 00:00:41.200 The rim of a tin helmet presses hard against your forehead. 00:00:41.200 --> 00:00:45.119 In your numb hands you hold a bucket of water. 00:00:45.119 --> 00:00:51.039 You are looking up into the blackout, waiting for incendiary bombs to fall. 00:00:51.039 --> 00:01:02.880 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. 00:01:02.880 --> 00:01:06.640 Bombs do fall in the area. 00:01:06.640 --> 00:01:08.879 Windows shatter. 00:01:12.319 --> 00:01:14.879 But the observatory stands. 00:01:14.879 --> 00:01:26.400 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. 00:01:26.400 --> 00:01:31.280 Because you are a computer, and the Navy needs you. 00:01:31.280 --> 00:01:37.120 I'm Daina Bouquin, and this is Found in the Machine. 00:01:37.120 --> 00:01:44.879 Before the word computer meant a machine on a desk, it was a job title. 00:01:44.879 --> 00:01:47.599 Often the job title of a woman. 00:01:47.599 --> 00:01:57.519 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. 00:01:57.519 --> 00:02:04.400 On their desks are papers covered in columns of numbers, pages and pages of them. 00:02:04.400 --> 00:02:10.560 Hourly measurements of water levels taken at ports and harbors across Britain and beyond. 00:02:10.560 --> 00:02:13.199 Some of the data is decades old. 00:02:13.199 --> 00:02:17.199 Some arrived by courier this morning marked urgent. 00:02:17.199 --> 00:02:20.639 They are responsible for predicting the tides. 00:02:20.639 --> 00:02:24.560 They are working under a man named Arthur Doodson. 00:02:24.560 --> 00:02:30.240 Arthur is 53 years old, and he is a brilliant mathematician. 00:02:30.240 --> 00:02:32.240 He is also deaf. 00:02:32.240 --> 00:02:41.360 He cannot hear the German bombs exploding outside, and he cannot hear the metallic whirr of the machines in the other rooms. 00:02:41.360 --> 00:02:50.400 But in his own way, Arthur Doodson listens to the ocean better than anyone else on Earth. 00:02:50.400 --> 00:02:56.719 To Arthur, the tide is not chaos. 00:02:56.719 --> 00:02:58.800 It is a cord. 00:02:58.800 --> 00:03:08.639 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. 00:03:08.639 --> 00:03:13.840 Pull those notes apart and you have the variables that make the tide. 00:03:13.840 --> 00:03:17.759 Scientists realized this in the nineteenth century. 00:03:17.759 --> 00:03:26.560 They realized that if you could measure each of the influences that shape the water, you could predict what the ocean would do next. 00:03:26.560 --> 00:03:35.520 But you needed the celestial pulls, the local quirks of the coastline, the depth. 00:03:35.520 --> 00:03:38.879 You needed these harmonic constituents. 00:03:38.879 --> 00:03:47.520 Even if you had those measurements though, you couldn't do this work quickly, and not in your head. 00:03:47.520 --> 00:03:50.319 So they built mechanical marvels. 00:03:50.319 --> 00:03:53.599 The Bidston Observatory has two of them. 00:03:53.599 --> 00:04:02.719 One was the Bidston-Kelvin machine, a descendant of Lord Kelvin's original tide predictor design installed for Doodson in the 1920s. 00:04:02.719 --> 00:04:08.960 The other was the Roberts-Légé machine, built in 1906 and later acquired for Bidston. 00:04:08.960 --> 00:04:11.280 It weighs over a ton. 00:04:11.280 --> 00:04:14.240 They are analog computers. 00:04:14.240 --> 00:04:22.000 Glorious, intricate contrivances of shining brass and steel filled with wheels and gears and pulleys. 00:04:22.000 --> 00:04:25.040 The way they work is mesmerizing. 00:04:25.040 --> 00:04:30.800 There is a wire winding its way over and under dozens of pulley wheels. 00:04:30.800 --> 00:04:33.920 Each wheel represents a different force. 00:04:33.920 --> 00:04:40.720 The pull of the moon, the pull of the sun, the wobble of an orbit. 00:04:40.720 --> 00:04:45.920 The women set the dials and turn a crank to run the ocean forward in time. 00:04:45.920 --> 00:04:56.399 It takes four hours to draw a year, but the setup, the work to tell the machine where to start, takes days. 00:04:56.399 --> 00:05:01.199 So day after day the women look at pages of hourly water levels. 00:05:01.199 --> 00:05:09.360 They lay cardboard stencils with cutout holes over the data, multiplying the numbers that show through by specific weights. 00:05:09.360 --> 00:05:11.680 They do the differencing. 00:05:11.680 --> 00:05:13.680 They do the smoothing. 00:05:13.680 --> 00:05:17.439 Down in the basement they find the harmonic constants. 00:05:17.439 --> 00:05:22.319 Then they set the brass dials and crank the wheel. 00:05:22.319 --> 00:05:30.160 The wire pulls tight, moving a pen across a rotating paper drum, drawing a rolling line. 00:05:30.160 --> 00:05:35.839 The shape of a future ocean, and the time it will take to rise. 00:05:35.839 --> 00:05:45.519 Across the English Channel, German Field Marshal Erwin Rommel paces the sands of France. 00:05:45.519 --> 00:05:48.480 He looks out at the water. 00:05:48.480 --> 00:05:50.639 The tides are massive. 00:05:50.639 --> 00:05:57.839 The vertical distance from low to high tide exceeds six meters, nearly twenty feet. 00:05:57.839 --> 00:06:02.240 When the tide comes in, it rushes at terrifying speed. 00:06:02.240 --> 00:06:06.160 The water rises roughly a foot every 15 minutes. 00:06:06.160 --> 00:06:11.360 Rommel tells his officers the Allies will come at high water. 00:06:11.360 --> 00:06:20.560 He cannot fathom that they would force soldiers to run across hundreds of yards of open, muddy beach under heavy machine gun fire. 00:06:20.560 --> 00:06:24.079 He prepares for a high tide invasion. 00:06:24.079 --> 00:06:27.279 He orders the beaches sewn with iron. 00:06:27.279 --> 00:06:41.279 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. 00:06:41.279 --> 00:06:45.519 Three-ton Belgian gates rolled into the shallows. 00:06:45.519 --> 00:06:57.680 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. 00:06:57.680 --> 00:07:00.639 To detonate when the ships come near. 00:07:00.639 --> 00:07:04.000 But Allied reconnaissance planes photograph the traps. 00:07:04.000 --> 00:07:08.959 The commanders realize that a high tide landing is a death trap. 00:07:08.959 --> 00:07:16.879 They realize they have to land at low tide, so the demolition engineers can see the steel obstacles and blow them to pieces. 00:07:16.879 --> 00:07:27.199 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. 00:07:27.199 --> 00:07:31.600 And they also need a bright, late rising moon. 00:07:31.600 --> 00:07:35.600 Otherwise the paratroopers won't be able to see their drop zones. 00:07:35.600 --> 00:07:38.560 And the daylight. 00:07:38.560 --> 00:07:44.800 They need an hour of daylight to bombard the coast before the troops hit the sand. 00:07:44.800 --> 00:07:49.199 Daylight has to coincide with low tide. 00:07:49.199 --> 00:07:51.519 And the invasion is huge. 00:07:51.519 --> 00:07:59.120 It spans five beaches Utah, Omaha, Gold, Juno, and Sword. 00:07:59.120 --> 00:08:02.959 The water sloshes through the English Channel like a funnel. 00:08:02.959 --> 00:08:09.120 The exact minute of low tide differs by more than an hour from one end of the invasion to the other. 00:08:09.120 --> 00:08:13.120 The landings have to be meticulously staggered. 00:08:13.120 --> 00:08:17.120 They need to know exactly what the water will do. 00:08:17.120 --> 00:08:20.959 They need someone to tell them when the earth will cooperate. 00:08:20.959 --> 00:08:26.959 They need the moon, the sun, the dawn, and the tides to align. 00:08:26.959 --> 00:08:37.120 On October 9th, 1943, Arthur Doodson receives a three-page handwritten letter marked MOST URGENT. 00:08:37.120 --> 00:08:44.480 It is from Commander William Ian Farquarson, the Admiralty's superintendent of tides. 00:08:44.480 --> 00:08:49.279 Farquarson cannot tell Doodson the biggest secret of the war. 00:08:49.279 --> 00:09:04.799 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. 00:09:04.799 --> 00:09:07.759 There is in fact very little data for it. 00:09:07.759 --> 00:09:16.799 I am gambling on the inferred shallow water constants giving something like the right answer." Doodson looks at the numbers. 00:09:16.799 --> 00:09:26.480 Constants inferred from sparse shallow water readings of nearby ports gathered by British reconnaissance teams in small boats and submarines in the night. 00:09:26.480 --> 00:09:33.279 Arthur knows the tides and guesses the location of Position Z, but he says nothing. 00:09:33.279 --> 00:09:36.000 He and the six women just get to work. 00:09:36.000 --> 00:09:41.600 They do the painstaking arithmetic, the harmonic analysis. 00:09:41.600 --> 00:09:44.159 They calibrate the brass wheels. 00:09:44.159 --> 00:09:46.000 They turn the crank. 00:09:46.000 --> 00:09:48.639 The pulleys rise and fall. 00:09:48.639 --> 00:09:53.120 They carefully translate the machine's raw output into tide tables. 00:09:53.120 --> 00:09:56.480 They map the future of a nameless coast. 00:09:56.480 --> 00:10:01.600 There are only three days in June 1944 that work. 00:10:01.600 --> 00:10:03.759 The fifth, the sixth, and the seventh. General Dwight D. 00:10:03.759 --> 00:10:09.360 Eisenhower chooses June 5th for the invasion, but a massive storm forces a delay. 00:10:09.360 --> 00:10:13.200 It doesn't matter what the tides do if the air won't cooperate. 00:10:13.200 --> 00:10:16.879 Rommel sees the same rough weather. 00:10:16.879 --> 00:10:21.519 He looks at the tides and concludes they are completely wrong for an invasion. 00:10:21.519 --> 00:10:28.399 Believing the Allies won't come, he leaves his headquarters and drives to Germany to celebrate his wife's birthday. 00:10:28.399 --> 00:10:31.360 But the Allies do come. 00:10:31.360 --> 00:10:42.240 On the morning of June 6, 1944, the largest amphibious invasion in the history of the world arrives on the coast of Normandy. 00:10:42.240 --> 00:10:45.600 The tide predictions hold. 00:10:45.600 --> 00:10:50.320 Water comes rushing in, rising and rising. 00:10:50.320 --> 00:10:57.840 On Omaha Beach, the American demolition teams have only minutes to blast their channels before the ocean swallows the traps. 00:10:57.840 --> 00:11:02.879 It is horrific. 00:11:02.879 --> 00:11:06.080 They knew it would be. 00:11:06.080 --> 00:11:17.039 Men in wool uniforms wading through cold water, weighed down by their packs, running toward the cliffs. 00:11:17.039 --> 00:11:19.039 Toward the wire. 00:11:19.039 --> 00:11:21.600 Toward the guns on the bluffs. 00:11:21.600 --> 00:11:24.799 Machine gun fire from the heights. 00:11:24.799 --> 00:11:30.080 The blast of heavy artillery, constant and deafening. 00:11:30.080 --> 00:11:39.360 Men waiting for their turn to run, sitting in small boats, breathing in engine fumes, watching their friends fall. 00:11:39.360 --> 00:11:45.759 More than half the engineers on Omaha are killed. 00:11:45.759 --> 00:11:53.440 And the fast rising water lifts the landing craft exactly as planned. 00:11:53.440 --> 00:12:01.679 They drop their ramps, they unload the troops, they reverse back into the channel. 00:12:01.679 --> 00:12:07.039 The tide does what they said it would do. 00:12:07.039 --> 00:12:25.279 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. 00:12:25.279 --> 00:12:40.960 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. 00:12:40.960 --> 00:12:51.759 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. 00:12:51.759 --> 00:12:54.399 But they did have names. 00:12:54.399 --> 00:13:07.679 And if you look hard, if you look really hard, you can find two of them: Joan Rossiter and Estelle Gilbert. 00:13:07.679 --> 00:13:18.240 Their names are mentioned in reference nine of a Physics Today article published in 2011, sixty-seven years after D-Day. 00:13:18.240 --> 00:13:23.600 The author had spoken to them a decade earlier and wrote down their names. 00:13:23.600 --> 00:13:26.960 The other four are unknown. 00:13:26.960 --> 00:13:36.240 We only know the names of some of the women who came later, and we only know them because they wrote the records themselves. 00:13:36.240 --> 00:13:40.320 They wrote down what it was like to operate the machines. 00:13:40.320 --> 00:13:44.559 They smiled in a group photo taken sometime after the war. 00:13:44.559 --> 00:13:50.799 Joan Rossiter and Estelle Gilbert are not in the photo. 00:13:50.799 --> 00:13:58.399 The women who helped calculate the tides in 1944 were swallowed by a job title. 00:13:58.399 --> 00:14:00.480 They were just Computers. 00:14:00.480 --> 00:14:04.399 They were diligent and good with numbers. 00:14:04.399 --> 00:14:07.200 They had neat handwriting. 00:14:07.200 --> 00:14:16.480 And when the tide came in on the morning of June 6th, 1944, they were already working on the next tide. 00:14:16.480 --> 00:14:18.480 And the one after that. 00:14:18.480 --> 00:14:20.960 Because a war was on. 00:14:20.960 --> 00:14:23.279 And the Navy needed them. 00:14:23.279 --> 00:14:31.759 I'm Daina Bouquin, and this is Found in the Machine. 00:14:31.759 --> 00:14:41.440 If you want to support this show, leave a rating or a review wherever you listen. 00:14:41.440 --> 00:14:44.480 Or share this episode with a friend. 00:14:44.480 --> 00:15:03.759 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. 00:15:03.759 --> 00:15:06.240 Thanks for listening.