John Henry Lefroy

Sir John Henry Lefroy KCMG CB FRS (28 January 1817 – 11 April 1890) was an English military man and later colonial administrator who also distinguished himself with his scientific studies of the Earth's magnetism.

Lefroy entered the Royal Military Academy, Woolwich in London in 1831 and became a 2nd lieutenant of the Royal Artillery in 1834. When the British government launched a project under the direction of Edward Sabine to study terrestrial magnetism, he was chosen to set up and supervise the observatory on Saint Helena. He embarked on 25 September 1839, for Saint Helena, and carried out his task throughout the following year. In 1842, Lefroy was sent to Toronto as the superintendent of the new Toronto Magnetic and Meteorological Observatory built there as part of that project. He immediately began planning a field expedition to the Canadian northwest to measure magnetism there. With an assistant and a Hudson's Bay Company brigade, he travelled more than 5,000 miles in the Northwest from May 1843 to November 1844, taking measurements at over 300 stations in an attempt to map the geo-magnetic activity of British North America from Montreal to the Arctic Circle, and locate the North Magnetic Pole. They followed the Mackenzie River as far as Fort Good Hope and visited Fort Simpson in the west. On 9 June 1848, Lefroy was made a member of the Royal Society.

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