Eminent Scotsman John Buchan (1875-1940) was an author of what, among other more sober works, he called “shockers”, such as The Thirty-Nine Steps. He described them as fictions “where the incidents defy the probabilities, and march just inside the borders of the possible”.

Indeed, as Dr Tom Greenslade, one of Buchan’s characters in The Three Hostages, put it, “Take three things a long way apart [...] invent a connection [and] weave all three into [a] yarn. The reader, who knows nothing about the three at the start, is puzzled [...] intrigued [and] finally satisfied [...] for he doesn’t realise that the author fixed upon the solution first, and then invented a problem to suit it.”

Sometimes the real world has done the author’s work and there’s no need to invent either the apparently unconnected facts or the narrative that links them. Take, for example, 19th-century naval architecture, anti-slavery patrols in the Atlantic Ocean and what is known as the Principal Datum, from which all heights in Hong Kong are measured.

The first piece in the puzzle is the story of the revolution that transformed ship design and building during the 19th century. Ships that had long depended on naturally occurring materials such as wood and canvas, and the energy of wind and muscle, became the steam-powered, screw-propelled, iron and, later, steel vessels on which the prosperity of ports such as Hong Kong was built. Suck-it-and-see design methods and the experienced eye of the traditional craftsman were replaced by the mathe­mat­ically modelled, tank-tested designs of technically trained profes­sional naval architects.

Anti-slavery patrols by ships of the British Royal Navy’s West Africa Squadron, also known as the Preventative Squadron, were conducted between 1818 and 1870. Based out of Freetown, in Sierra Leone, the newly created future home for liberated slaves, the squadron always struggled. It had too few ships, the constraints of maritime law to contend with and too much ocean to cover. Even so, by the time it ceased to exist, 1,600 slave ships had been captured, 150,000 slaves freed and the trade in slaves effectively ended.

In the beginning, the squadron had also been hampered by the dependence of sailing ships on the wind – a commodity always in fitful supply in equatorial regions. In the 1840s, though, the squadron began receiving its first paddle steamers and, later, screw steamers, and the navy’s patrols were released from that dependence. Among the latter, HMS Rifleman arrived on station in 1848.

The final piece in the puzzle can be found at the Hong Kong Maritime Museum. For more than a century the exhibit in question was the city’s Principal Datum: a fixed vertical point from which all levels in Hong Kong were measured, from the heights of buildings and mountains to the depths of foundations and MTR tunnels. It’s in the Principal Datum’s old name, and the ship it recollects, that our Buchanesque connections are to be found.

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Two men straddle the 50-year period that transformed naval architecture. First came John Fincham (1785-1859), one of the last of the old master shipwrights and a precursor of the new breed of naval architect. Master shipwright of Britain’s Royal Dockyard at Portsmouth from 1844 to 1852, he was a major proponent of screw propulsion. In 1845, he designed and supervised the building of the screw steam gunboat HMS Rifleman.

Then comes one of Fincham’s pupils, Edward James Reed (1830-1906), the first fully fledged professional naval architect to be put in charge of ship design and con­struction for the Royal Navy. Reed had a part in the founding of the Institute of Naval Architects (now the Royal Institution of Naval Architects) in 1860, becoming its first secretary, and was appointed the Royal Navy’s chief constructor – the navy’s top warship architect – in 1863. With Reed we see the establish­ment of a new profession that would underpin the late 19th-century explosion in seaborne trade.

At the heart of our story, however, is the ship. The Rifleman spent the first decade of her career, from 1848 until she was paid off, in 1857, chasing slavers between West Africa and South America. By the time she returned to Woolwich, in London, in September 1857, the Rifleman’s timbers were rotten and she was clapped out.

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Instead of scrapping the ship, however, the Royal Navy, short of funds as ever during this period, did what it seems to have done with a number of other small, obsolete war­ships over the years. It had the Rifleman refitted, its engines removed and a new ship rig of three masts fitted to ready her for work with the Royal Navy’s Surveying Service. Recommissioned on November 14, 1861, the Rifleman was sent on survey duty to the China seas.

In charge of the Rifleman on its new service in the China seas was John William Reed, Master Royal Navy. Little is known about John Reed’s early life, save that he was born in 1826 and joined the navy as a clerk around 1840. Changing tracks to become a master’s assistant, he was promoted to second master in July 1851. By 1853, he was in the Surveying Service, aboard HMS Saracen, working the Fujian coast, the west coast of Taiwan, Korea and Japan. By 1856, the Saracen was in the Gulf of Thailand and Reed was an assistant surveyor. He served until March 1860, when the ship was deemed unfit for service.

Back in Britain, Reed swapped the steamy Gulf of Thailand for the less balmy delights of the North Atlantic, Labrador, Greenland and the River Mersey. Then, in November 1861, he was given command of the refitted and recommissioned Rifleman and dispatched to East Asia. Arriving in Singapore in 1862, the Rifleman worked Southeast Asian waters until, in 1864, Reed was invalided home.

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When Reed returned to the Rifleman in early 1866, one of his first jobs was to undertake a detailed survey of the north shore of Hong Kong Island. And it’s this that brings us to the Principal Datum, for before he could carry out the survey, Reed had a problem to solve: he needed a fixed point from which to measure his surroundings – the heights that would appear on Hong Kong’s terrestrial maps. The datum then in use had been established in 1841 by Commander Edward Belcher, in somewhat of a hurry – and it was not entirely accurate.

To establish the datum, the Rifleman’s lads set a copper spike into a hole in a solid granite cornerstone of one of the Naval Yard’s new store blocks. The Ordnance or Colony Datum, as it was first called, was established as 5.435 metres below the centre of the bolt. For the next 119 years, with a change of location when the old Naval Yard buildings were bulldozed to make way for the Admiralty complex in the late 1960s, what had become known as the Rifleman’s Bolt remained one of Hong Kong’s key fixed points.

So we have the pieces of our puzzle in the shape of naval architecture, represented by Edward Reed and Fincham, the ex-slaver hunter Rifleman and her surveying skipper, and the Rifleman’s Bolt. But how do they all fit together?

The answer, if you hadn’t already guessed it, is to be found in the name Reed. Extensive archival research has shown for the first time that Sir Edward James Reed and John William Reed were in fact brothers.

Born in Sheerness, near Chatham, in Kent, southern England, Edward and John Reed were the sons of a naval dockyard shipwright. John was the oldest of five brothers and, as we have seen, joined the navy early on. Edward, the third son, was appren­ticed in the same dockyard where his father worked in about 1844 and in around 1849 was selected for the Central School of Mathematics and Naval Construction, in Portsmouth – the only British institution offering professional training in what was still often called “naval science”. Someone had spotted Edward’s potential.

Fincham was responsible for shipbuilding classes at the school and Edward was among his students.

The stories of the Reed brothers illustrate well the way in which Victorian society was more receptive to talent than is often supposed. That two gifted sons of a humble dockyard work­er were able to rise and make their respective marks – marks that, for all the defects of the colonial period, were to benefit the maritime world in general, and Hong Kong in particular – is telling of the age.

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Sir Edward James Reed, KCB, FRS, was evidently the star. Having designed ships for the Royal Navy and the Japanese, German, Brazilian and Chilean navies, he also authored major texts in naval architecture and a popular history-cum-travelogue of Japan, became a member of parliament and made a name as a Florida railroad magnate.

One of his designs for the Chilean navy, the pre-dread­nought battleship Constitucion, provides another Hong Kong connection. Eventually bought for the Royal Navy, it was renamed HMS Triumph. In 1913, the obsolescent Triumph was designated the successor of HMS Tamar as the Royal Navy base depot ship and arrived in Hong Kong in August that year. Before her conversion was complete and the Tamar turned into razor blades, the first world war broke out. The Triumph was put back into service, helped the Japanese capture Qingdao and was sunk off Gallipoli, Turkey, in May 1915.

John Reed might not have achieved his brother’s public stature or wealth, but his work on the second, 1867 edition of the Admiralty’s China Sea Pilot and the many surveys he made in the China seas have ensured that his name lives on, as in Reed Bank, a tablemount close to (or part of, some argue) the South China Sea’s Spratly Islands.

And the fate of the Rifleman? When John Reed’s health broke down once more in 1868 and he returned to Britain, the poor old ship was condemned as unfit for service. Sold in Hong Kong on November 18, 1869, she was broken up. Fittingly, John Reed had immortalised the Rifleman, too, not just with the Rifleman’s Bolt, but also with Rifleman Bank, amid the Spratly Islands, and Rifleman Rock, off the northeast coast of Sabah, Malaysia.

John Reed did not long outlast his last ship. After the better part of a decade working in climes infested with malaria, dengue fever and cholera, he died in London on November 18, 1873, five days before his 46th birthday.

In Hong Kong, the Rifleman’s Bolt bears witness to all of this. Go and pay it a visit some time.