Lefroy was a son of the Rev. John Henry George Lefroy, of Ewshot House (subsequently Itchel) in Hampshire, England, and his wife, Sophia Cottrell. His sister Anne married the Irish landowner and politician John McClintock, who was created the 1st
Baron Rathdonnell in 1868. Lefroy was also a first cousin to
Thomas Lefroy (1776-1869), the future Chief Justice of Ireland. 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, Later, he became
Inspector General of army schools and finally in 1868 director of the Ordnance Office. In 1859 his wife died, and on 12 September the following year he married his second wife Charlotte Anna
née Dundas (widow of Col. Armine Mountain). from 21 October 1880 to 7 December 1881. John Henry Lefroy was made a Companion of the
Order of the Bath (CB) in 1870, and knighted in 1877 (
KCMG). == Legacy ==