Don't overlook how important cataloguing is. As the article mentions, Cotton (Robert Cotton 1571–1631), a proflific collector, also had Beowulf in his collection.
That remains the only source of Beowulf known to exist - in fact, nobody heard of Beowulf (for centuries) until Cotton's manuscript of it was found. Beowulf is essential to history - not only of England, but Denmark and Europe - the oldest and, in the first 1,100 years CE, the longest work in English. It's ~3,000 lines are about 10% of all surviving Old English poetry (or Anglo-Saxon, another word for Old English, after the Angles and Saxons who settled in England and brought new language).
When Cotton died in 1631, he hadn't heard of Beowulf either.[0] Cotton's collection wasn't catalogued (afaik) and nobody knew it was there. Cotton's child inherited the collection, and died without ever hearing of Beowulf, sitting right there on his shelves for his entire life, uncatalogued. Cotton's grandchild, John Cotton toward the end of life in 1700 donated the collection to the country - but didn't know they were donating something called Beowulf.
In 1705, a Humphrey Wanley had a go at cataloguing Cotton's donation and found Beowulf ... but Wanley didn't look carefully, didn't understand what they had found, and wrote a misleading description. Wanley did transcribe a few lines, which was important later. Wanley's cataloguing turned out to be essential, however ...
As of October 23rd, 1731 Beowulf remained, for any practical purpose, undiscovered. It was stored with the rest of the Cotton collection at Ashburnham House in Westminster, and on that day, Ashburnham House caught fire ...
How many Beowulfs were lost - and will never even be known thanks to poor cataloguing? Maybe if the firefighters had approached from a different direction, or the people rescuing manuscripts started in a different room, we'd be talking about a different manuscript - or no manuscript at all - and never use the word Beowulf. But in this case, in this reality, Beowulf is one of the manuscripts that was saved, its edges badly scorched and suffering other damage too.
But nobody knew it. Still it was unknown. And its identity and value unknown, and the 700+ year old parchment now having been through a fire, it is unprotected and starts crumbling around the edges. Text is being lost, text we will never know (unless someone finds and catalogues another lost manuscript).
In 1777, Jakob Langebek, Danish National Archivist (the Danes naturally have a strong interest in Anglo-Saxon history of England) discovers not Beowulf (or there's no evidence of that) but Wanley's catalogue description of it. Langebek notes Beowulf's existence, "marvelling that no Englishman had yet produced an edition". No Englishperson gets the hint.
On 3 October 1786, Grímur Jónsson Thorkelín, another leading Danish archivist researching in Britain, is at the British Library and fits between lines in his notes, "xv See Wanlej in Hickes". Thorkelin has found Beowulf - thanks to Wanley's catalogue ("xv" is Cotton's number for the book in which Beowulf is bound, with some other manuscripts), but still hasn't identified it. At some point Thorkelin has someone in the British Library make a transcript of it, perhaps for future study back in Denmark. This transcript, 'Thorkelin A', is the earliest complete transcript (and you now can see why Wanley's short transcription in 1705, before the fire, was valuable). As the manuscript continued to crumble, Thorkelin A records text that is now lost.
What did Thorkelin know at this point? A month later he wrote in a letter, "among the manuscripts are not a few pertaining to Denmark; I hope to find something worthwhile, including an unknown Anglo-Saxon [work], containing a collection of songs about the exploits of the Danish kings in the 3rd century, and by which the good Saxo is increased not a little." The summary apparently matches Wanley's short transcription, so there's no sign that Thorkelin had read the poem yet. By 1791 Thorkelin made another transcription, now known as "Thorkelin B", and returned to Denmark.
I'm not sure when Thorkelin actually realized what he had (it may be well-known, but not to me), but this much is known: Thorkelin sat on it. He did not publish, he did not share his discovery with the world. Beowulf remained, effectively, lost.
In 1805, Sharon Turner wrote the first published mention of the poem, calling it "a composition most curious and important.... [I]t may be called an Anglo-Saxon epic poem." Turner even translated an excerpt - the first translation into modern English. I don't know why Turner didn't go further, and still Thorkelin sat on it. [1]
In 1815, Thorkelin published De Danorum Rebus Gestis Secul. III & IV. Poëma Danicum Dialecto Anglosaxonica. Ex Bibliotheca Cottoniana Musaei Britannici, at last sharing Beowulf with the world, and the world ran with it. Thorkelin's scholarship and Latin translation are long forgotten, but without Wanley's catalogue, and Thorkelin's discovery and eventual publishing, we might never know.
(In 1845 the British Library mounted the manuscript in frames to protect it from further damage.)
______________________________
You can find the Beowulf manuscript in its current state, the Thorkelin A and Thorkelin B transcripts, and compare them all side-by-side at Electronic Beowulf: https://ebeowulf.uky.edu/ebeo4.0/CD/main.html
Translations: Electronic Beowulf has an option to enable translations in popups for each word or each line. For more usable and complete translations, Seamus Heaney (2000) is more evocative, Roy Liuzza (1999 / 2012) is more accurate but still very readable. Older leading translations are E. Talbot Donaldson (1966/1975), Francis B. Gummere (verse, and out of copyright). JRR Tolkien, a leading Old English scholar in his day job, wrote a translation but didn't publish it; it was eventually published, with Tolkien's scholarly commentary on the words and their meaning, in ~2014. Note that Old English is very different in style from modern English; any translation that's accurate will seem alien in many ways.
A detailed complete story of Thorkelin and source of quotes: "Part One: Thorkelin's Discovery of Beowulf" by Kevin Kiernan, a leading Beowulf scholar (and the person behind Electronic Beowulf).
https://www.uky.edu/~kiernan/Thorkelin/Th_1/
[0] The Beowulf manuscript was written by two scribes somewhere around the turn of the millenium (very approximately and much debated). It vanished from history and reappeared in 1563, when a Laurence Nowell, antiquary, wrote that date in their new piece, but Nowell didn't apparently realize what they had. Cotton ended up with some time in the 17th century.
[1] Some say Thorkelin sat on it because such scholarship was his weak point, and he did not want to be revealed.
That remains the only source of Beowulf known to exist - in fact, nobody heard of Beowulf (for centuries) until Cotton's manuscript of it was found. Beowulf is essential to history - not only of England, but Denmark and Europe - the oldest and, in the first 1,100 years CE, the longest work in English. It's ~3,000 lines are about 10% of all surviving Old English poetry (or Anglo-Saxon, another word for Old English, after the Angles and Saxons who settled in England and brought new language).
When Cotton died in 1631, he hadn't heard of Beowulf either.[0] Cotton's collection wasn't catalogued (afaik) and nobody knew it was there. Cotton's child inherited the collection, and died without ever hearing of Beowulf, sitting right there on his shelves for his entire life, uncatalogued. Cotton's grandchild, John Cotton toward the end of life in 1700 donated the collection to the country - but didn't know they were donating something called Beowulf.
In 1705, a Humphrey Wanley had a go at cataloguing Cotton's donation and found Beowulf ... but Wanley didn't look carefully, didn't understand what they had found, and wrote a misleading description. Wanley did transcribe a few lines, which was important later. Wanley's cataloguing turned out to be essential, however ...
As of October 23rd, 1731 Beowulf remained, for any practical purpose, undiscovered. It was stored with the rest of the Cotton collection at Ashburnham House in Westminster, and on that day, Ashburnham House caught fire ...
How many Beowulfs were lost - and will never even be known thanks to poor cataloguing? Maybe if the firefighters had approached from a different direction, or the people rescuing manuscripts started in a different room, we'd be talking about a different manuscript - or no manuscript at all - and never use the word Beowulf. But in this case, in this reality, Beowulf is one of the manuscripts that was saved, its edges badly scorched and suffering other damage too.
But nobody knew it. Still it was unknown. And its identity and value unknown, and the 700+ year old parchment now having been through a fire, it is unprotected and starts crumbling around the edges. Text is being lost, text we will never know (unless someone finds and catalogues another lost manuscript).
In 1777, Jakob Langebek, Danish National Archivist (the Danes naturally have a strong interest in Anglo-Saxon history of England) discovers not Beowulf (or there's no evidence of that) but Wanley's catalogue description of it. Langebek notes Beowulf's existence, "marvelling that no Englishman had yet produced an edition". No Englishperson gets the hint.
On 3 October 1786, Grímur Jónsson Thorkelín, another leading Danish archivist researching in Britain, is at the British Library and fits between lines in his notes, "xv See Wanlej in Hickes". Thorkelin has found Beowulf - thanks to Wanley's catalogue ("xv" is Cotton's number for the book in which Beowulf is bound, with some other manuscripts), but still hasn't identified it. At some point Thorkelin has someone in the British Library make a transcript of it, perhaps for future study back in Denmark. This transcript, 'Thorkelin A', is the earliest complete transcript (and you now can see why Wanley's short transcription in 1705, before the fire, was valuable). As the manuscript continued to crumble, Thorkelin A records text that is now lost.
What did Thorkelin know at this point? A month later he wrote in a letter, "among the manuscripts are not a few pertaining to Denmark; I hope to find something worthwhile, including an unknown Anglo-Saxon [work], containing a collection of songs about the exploits of the Danish kings in the 3rd century, and by which the good Saxo is increased not a little." The summary apparently matches Wanley's short transcription, so there's no sign that Thorkelin had read the poem yet. By 1791 Thorkelin made another transcription, now known as "Thorkelin B", and returned to Denmark.
I'm not sure when Thorkelin actually realized what he had (it may be well-known, but not to me), but this much is known: Thorkelin sat on it. He did not publish, he did not share his discovery with the world. Beowulf remained, effectively, lost.
In 1805, Sharon Turner wrote the first published mention of the poem, calling it "a composition most curious and important.... [I]t may be called an Anglo-Saxon epic poem." Turner even translated an excerpt - the first translation into modern English. I don't know why Turner didn't go further, and still Thorkelin sat on it. [1]
In 1815, Thorkelin published De Danorum Rebus Gestis Secul. III & IV. Poëma Danicum Dialecto Anglosaxonica. Ex Bibliotheca Cottoniana Musaei Britannici, at last sharing Beowulf with the world, and the world ran with it. Thorkelin's scholarship and Latin translation are long forgotten, but without Wanley's catalogue, and Thorkelin's discovery and eventual publishing, we might never know.
(In 1845 the British Library mounted the manuscript in frames to protect it from further damage.)
______________________________
You can find the Beowulf manuscript in its current state, the Thorkelin A and Thorkelin B transcripts, and compare them all side-by-side at Electronic Beowulf: https://ebeowulf.uky.edu/ebeo4.0/CD/main.html
Translations: Electronic Beowulf has an option to enable translations in popups for each word or each line. For more usable and complete translations, Seamus Heaney (2000) is more evocative, Roy Liuzza (1999 / 2012) is more accurate but still very readable. Older leading translations are E. Talbot Donaldson (1966/1975), Francis B. Gummere (verse, and out of copyright). JRR Tolkien, a leading Old English scholar in his day job, wrote a translation but didn't publish it; it was eventually published, with Tolkien's scholarly commentary on the words and their meaning, in ~2014. Note that Old English is very different in style from modern English; any translation that's accurate will seem alien in many ways.
A detailed complete story of Thorkelin and source of quotes: "Part One: Thorkelin's Discovery of Beowulf" by Kevin Kiernan, a leading Beowulf scholar (and the person behind Electronic Beowulf). https://www.uky.edu/~kiernan/Thorkelin/Th_1/
[0] The Beowulf manuscript was written by two scribes somewhere around the turn of the millenium (very approximately and much debated). It vanished from history and reappeared in 1563, when a Laurence Nowell, antiquary, wrote that date in their new piece, but Nowell didn't apparently realize what they had. Cotton ended up with some time in the 17th century.
[1] Some say Thorkelin sat on it because such scholarship was his weak point, and he did not want to be revealed.