The Northern line is a London Underground line that runs from south-west to north-west London, with two branches through central London and three in north London. It runs northwards from its southern terminus at Morden in the borough of Merton to Kennington in Southwark, where it divides into two central branches, one via Charing Cross in the West End and the other via Bank in the City; the central branches re-join at Camden Town where the line again divides into two branches, one to High Barnet and the other to Edgware in the borough of Barnet. The High Barnet branch has an additional single-station spur at Finchley Central with a shuttle train to Mill Hill East. For most of its length it is a deep-level tube line; the portion between Stockwell and Borough opened in 1890 and is the oldest section of deep-level tube line on the Underground network. About 294 million passenger journeys were recorded in 2016/17 on the Northern line, making it the busiest on the Underground, it is unique in having two different routes through two northern branches.

Despite its name, it does not serve the northernmost stations on the network, though it does serve the southernmost station, Morden, as well as 16 of the system's 29 stations south of the River Thames. There are 50 stations in total on the line; the line has a complicated history, the current complex arrangement of two main northern branches, two central branches and the southern route reflects its genesis as three separate railways, combined in the 1920s and 1930s. An extension in the 1920s used a route planned by a fourth company. Abandoned plans from the 1920s to extend the line further southwards, northwards in the 1930s, would have incorporated parts of the routes of two further companies. From the 1930s to the 1970s, the tracks of a seventh company were managed as a branch of the Northern line. An extension of the Charing Cross branch from Kennington to Battersea is under construction, which may either give the Northern line a second southern branch or may see it split into separate distinct lines with their own identities.

It is coloured black on the current Tube map. See City and South London Railway and Charing Cross and Hampstead Railway for detailed histories of these companies The core of the Northern line evolved from two railway companies: the City & South London Railway and the Charing Cross, Euston & Hampstead Railway; the C&SLR, London's first deep-level tube railway, was built under the supervision of James Henry Greathead, responsible, with Peter W. Barlow, for the Tower Subway, it was the first of the Underground's lines to be constructed by boring deep below the surface and the first to be operated by electric traction. The railway opened in November 1890 from Stockwell to a now-disused station at King William Street; this was inconveniently placed and unable to cope with the company's traffic so, in 1900, a new route to Moorgate via Bank was opened. By 1907 the C&SLR had been further extended at both ends to run from Clapham Common to Euston; the CCE&HR was opened in 1907 and ran from Charing Cross via Euston and Camden Town to Golders Green and Highgate.

It was extended south by one stop to Embankment in 1914 to form an interchange with the Bakerloo and District lines. In 1913 the Underground Electric Railways Company of London, owner of the CCE&HR, took over the C&SLR, although they remained separate companies. During the early 1920s, a series of works was carried out to connect the C&SLR and CCE&HR tunnels to enable an integrated service to be operated; the first of these new tunnels, between the C&SLR's Euston station and the CCE&HR's station at Camden Town, had been planned in 1912 but had been delayed by World War I. The second connection linked the CCE&HR's Embankment and C&SLR's Kennington stations and provided a new intermediate station at Waterloo to connect to the main line station there and the Bakerloo line; the smaller-diameter tunnels of the C&SLR were expanded to match the standard diameter of the CCE&HR and the other deep tube lines. In conjunction with the works to integrate the two lines, two major extensions were undertaken: northwards to Edgware in Middlesex and southwards to Morden in Surrey.

The Edgware extension used plans dating back to 1901 for the Edgware and Hampstead Railway which the UERL had taken over in 1912. It extended the CCE&HR line from its terminus at Golders Green to Edgware in two stages: to Hendon Central in 1923 and to Edgware in 1924; the line crossed open countryside and ran on the surface, apart from a short tunnel north of Hendon Central. Five new stations were built to pavilion-style designs by Stanley Heaps, head of the Underground's Architects Office, stimulating the rapid northward expansion of suburban developments in the following years; the engineering of the Morden extension of the C&SLR from Clapham Common to Morden was more demanding, running in tunnels to a point just north of Morden station, constructed in a cutting. The line runs under the wide station forecourt and public road outside the station, to the depot; the extension was planned to continue to Sutton over part of the route for the unbuilt Wimbledon and Sutton Railway, in which the UERL held a stake, but agreements were made with the Southern Railway to end the extension at Morden.

The Southern Railway built the surface line from Wimbledon to Sutton, via South Merton and St. Helier; the tube extension opened in 1926, with seven new stations, all designed by Charles Holden

Karl Christoph Traugott Tauchnitz was a German printer and bookseller. He was born at Grosspardau, near Leipzig, Germany, he learned the printer's trade at Leipzig, worked in the printing house of Unger in Berlin. In 1792 he entered the house of Sommer in Leipzig, he began a small printing business of his own in Leipzig in 1796. In 1798 he opened a bookstore in connection with the printing business, in 1800 a type foundery, his business, “Karl Tauchnitz,” became one of the largest establishments of the kind in Germany. In 1809 he began to issue Greek and Latin classics in accurate and cheap editions, they circulated throughout Europe, he published fine editions of classical authors in folio. By offering a prize of a ducat for every error pointed out, he brought out a remarkably correct edition of Homer. In 1816 he introduced stereotyping into Germany and applied it to music, an experiment which had not been tried before, his edition of Mozart's Don Giovanni had a wide popularity. His stereotyped editions of the classics were once famed alike for their cheapness, their convenience, their accuracy.

He printed stereotype editions of oriental works, including two of the Hebrew Bible, an edition of the Koran. By his will Leipzig received 4,500,000 marks for charitable ends. A son, Karl Christian Philipp Tauchnitz, carried on the business. A Tauchnitz edition of English authors, which numbered over 3,700 volumes, was begun in 1842 by a nephew, Christian Bernhard, Freiherr von Tauchnitz. Tauchnitz publishers

SS Great Britain is a museum ship and former passenger steamship, advanced for her time. She was the longest passenger ship in the world from 1845 to 1854, she was designed by Isambard Kingdom Brunel, for the Great Western Steamship Company's transatlantic service between Bristol and New York. While other ships had been built of iron or equipped with a screw propeller, Great Britain was the first to combine these features in a large ocean-going ship, she was the first iron steamer to cross the Atlantic, which she did in the time of 14 days. The ship has a 3,400-ton displacement, she was powered by two inclined 2 cylinder engines of the direct-acting type, with twin high pressure cylinders and twin low pressure cylinders 88 in bore, all of 6-foot stroke cylinders. She was provided with secondary masts for sail power; the four decks provided accommodation for a crew of 120, plus 360 passengers who were provided with cabins, dining and promenade saloons. When launched in 1843, Great Britain was by far the largest vessel afloat.

But her protracted construction time of six years and high cost had left her owners in a difficult financial position, they were forced out of business in 1846, having spent all their remaining funds refloating the ship after she ran aground at Dundrum Bay in County Down near Newcastle in what is now Northern Ireland, after a navigation error. In 1852 she was repaired. Great Britain carried thousands of immigrants to Australia from 1852 until being converted to all-sail in 1881. Three years she was retired to the Falkland Islands, where she was used as a warehouse, quarantine ship and coal hulk until she was scuttled and sunk in 1937, 98 years since being laid down at the start of her construction. In 1970, after Great Britain had lain under water and abandoned for 33 years, Sir Jack Arnold Hayward, OBE paid for the vessel to be raised and repaired enough to be towed north through the Atlantic back to the United Kingdom, returned to the Bristol dry dock where she had been built 127 years earlier.

Hayward was a prominent businessman, developer and owner of the English football club Wolverhampton Wanderers. Now listed as part of the National Historic Fleet, Great Britain is a visitor attraction and museum ship in Bristol Harbour, with between 150,000 and 200,000 visitors annually. After the initial success of its first liner, SS Great Western of 1838, the Great Western Steamship Company collected materials for a sister ship, tentatively named City of New York; the same engineering team that had collaborated so on Great Western—Isambard Brunel, Thomas Guppy, Christopher Claxton and William Patterson—was again assembled. This time however, whose reputation was at its height, came to assert overall control over design of the ship—a state of affairs that would have far-reaching consequences for the company. Construction was carried out in a specially adapted dry dock in England. Two chance encounters were to profoundly affect the design of Great Britain. In late 1838, John Laird's 213-foot English Channel packet ship Rainbow—the largest iron-hulled ship in service—made a stop at Bristol.

Brunel despatched his associates Christopher Claxton and William Patterson to make a return voyage to Antwerp on Rainbow to assess the utility of the new building material. Both men returned as converts to iron-hulled technology, Brunel scrapped his plans to build a wooden ship and persuaded the company directors to build an iron-hulled ship. Great Britain's builders recognised a number of advantages of iron over the traditional wooden hull. Wood was becoming more expensive. Iron hulls were not subject to dry rot or woodworm, they were lighter in weight and less bulky; the chief advantage of the iron hull was its much greater structural strength. The practical limit on the length of a wooden-hulled ship is about 300 feet, after which hogging—the flexing of the hull as waves pass beneath it—becomes too great. Iron hulls are far less subject to hogging, so that the potential size of an iron-hulled ship is much greater; the ship's designers, led by Brunel, were cautious in the adaptation of their plans to iron hulled-technology.

With each successive draft however, the ship grew larger and bolder in conception. By the fifth draft, the vessel had grown to 3,400 tons, over 1,000 tons larger than any ship in existence. In early 1840, a second chance encounter occurred, the arrival of the revolutionary SS Archimedes at Bristol, the first screw-propelled steamship, completed only a few months before by F. P. Smith's Propeller Steamship Company. Brunel had been looking into methods of improving the performance of Great Britain's paddlewheels, took an immediate interest in the new technology. Smith, sensing a prestigious new customer for his own company, agreed to lend Archimedes to Brunel for extended tests. Over several months and Brunel tested a number of different propellers on Archimedes to find the most efficient design, a four-bladed model submitted by Smith. Having satisfied himself as to the advantages of screw propulsion, Brunel wrote to the company directors to persuade them to embark on a second major design change, abandoning the paddlewheel engines—already half constructed—for new engines suitable for powering a propeller.

Brunel listed the advantages of the screw propeller over the paddlewheel as follows: Screw propulsion machinery was lighter in weight, thus improving fuel economy.