ABSTRACT
The fundamental methods of determining position at sea involve the measurement of the altitudes of known celestial bodies the horizon, or of their positions relative to the neighbouring stars, by means of instruments capable of being effectively employed on board ship. Trials were made of Hadley’s instrument in 1732 on board an Admiralty yacht off Sheerness, in which Hadley himself took part, together with his brother Henry Hadley, and James Bradley. The claim of Thomas Godfrey to have invented an instrument for taking altitudes by means of a double reflection was brought before the Royal Society, in January 1734, in a letter from the American mechanician himself. The instrument embodied Le Roy’s inventions of the compensation balance, and of the detached chronometer escapement. The problem of constructing a marine chronometer of the required precision was first solved by John Harrison, a Yorkshire-man who early showed genius in the construction and improvement of clocks.
