Armstrong, Sir Alexander (1818–99), naval surgeon and explorer, was born in Co. Donegal, son of Alexander Armstrong of Croghan Lodge, Co. Fermanagh. His family was originally from Cumberland and one of his ancestors was Maj.-gen. John Armstrong (d. 1742), a military engineer and ADC to the 1st duke of Marlborough (qv).
He entered TCD to study medicine, later moving to Edinburgh, where in 1841 he graduated MD and became a licentiate of the Royal College of Surgeons, Edinburgh. In 1842 he entered the Royal Navy as an assistant surgeon and initially served at the Haslar Hospital in Gosport and aboard the flagship in Portsmouth before being appointed to HMS Polyphemus, then serving in the Mediterranean. During his early service he proved himself a gifted and progressive surgeon. He served as the medical officer to a party exploring Xante (Xanthus) in Lycia, and was commended by the admiralty for his sanitary arrangements during this expedition (1843–6). The trustees of the British Museum also later passed a vote of thanks for the scientific observations he had maintained throughout this expedition. In 1846 he was appointed to HMS Grappler, which was preparing for an expedition to West Africa. He did not go on this expedition, however, as he was appointed to the royal yacht and served aboard during Queen Victoria's visit to Ireland in 1849, being promoted to surgeon in October of that year.
In late 1849 he was appointed as the surgeon aboard HMS Investigator, which was then preparing for an Arctic expedition under the command of Commander (later Vice-adm.) Robert John Le Mesurier McClure (qv). The expedition was intended to search for Sir John Franklin, who had been missing with two ships and over 120 men since 1845. In January 1850 the Investigator left England in company with HMS Enterprise under the overall command of Capt. Richard Collinson. From the outset, Armstrong's relationship with McClure was difficult. The other members of the wardroom subjected him to rough horseplay, which McClure made no effort to stop. When McClure broke away from Collinson to make a personal attempt on the North West Passage, Armstrong criticised him for veering from established practices in Arctic exploration. During the next three years Armstrong proved himself an excellent surgeon, giving the crew infusions of lime juice to delay the onset of scurvy. In September 1851 the Investigator became beset in the Bay of Mercy on Banks Island, where it would remain. The crews were put on reduced rations and it is testimony to Armstrong's skill that only six men died. However, by the time a rescue party from the Resolute, commanded by Capt. Henry Kellett (qv), arrived in April 1853, two men had gone insane while the remainder of the crew were suffering from the onset of scurvy and the results of a near-starvation diet. Armstrong had earlier opposed McClure's plans to send out parties on an overland expedition, and he now stated that the crew were not fit for further service. The Investigator was abandoned and the crew evacuated to England.
After being paid off in October 1854, the petty officers and men of the Investigator presented Armstrong with an address of thanks and a gold watch and chain. He was also awarded the Arctic Medal. During 1855 he served aboard the Cornwallis in the Baltic and was the senior naval medical officer present at the battle of Sweaborg. McClure had meantime successfully made his case for having completed a transit of the North West Passage, while his journal was edited and published in 1856. Despite McClure having ordered all his officers to leave their journals behind, Armstrong had kept his. In 1857 he published A personal narrative of the discovery of the North West Passage. It was an honest and readable account of the expedition, and he was awarded the Gilbert Blaine gold medal for it. The Investigator expedition was also the longest expedition to that date, and he had kept a comprehensive medical journal throughout. In this he proved that lime juice could deter scurvy, and in 1858 he published Observations on naval hygiene and scurvy.
In July 1858 he was promoted to deputy inspector-general and subsequently served as the superintendent of the naval hospital at Malta (1859–64). He was appointed honorary surgeon to the prince of Wales (1863), and honorary surgeon (1866) and honorary physician (1870) to Queen Victoria. Promoted to inspector-general in 1866, he served as director-general of the navy's medical department (1869–80). In June 1871 he was created a KCB, and in 1872 was awarded an honorary doctorate from TCD. He was elected a fellow of the Royal Society (1873) and was also a fellow of the Royal College of Physicians and the Royal Geographical Society. He retired on a Royal Navy pension of £300 a year in February 1880.
He married (August 1894) Lady King Hall (née Jane Charlotte Simpson), widow of Adm. Sir William King Hall (d. 1886). He died at her residence, The Elms, Sutton-Bonington, Nottinghamshire, on 4 July 1899. There is an unsigned oil portrait of him at the Haslar Hospital in Gosport. Armstrong Point, on the west coast of Prince Albert Land in the Arctic, was named in his honour.