HISTORY OF PHYSICAL ASTRONOMY.

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He remarked that by observing the meridional altitude of that body neareach equinox, two times might be found when he had the same declina-tion ; and by the tables, the intermediate arc described by the sun mightbe found, half of whose excess above a semicircle (if it were more thanone), or half of whose defect (if it were less), would be his true distancefrom either equinox at those times *. This idea forms the germ of a fun-damental method of great importance which astronomy owes to Flam-steed, and which we shall presently have occasion to allude to more par-ticularly.

Finding by experience that the sextant was very ill adapted for deter-mining the meridional altitudes of the celestial bodies, and that theGovernment was not disposed to supply him with a mural arc, Flamsteedresolved to construct an instrument of the latter description at bis ownexpense. This design was carried into effect by him in the year 1681 ;but the instrument was not fixed in the meridian till 1083. It wasof the same radius as the sextant. The arc was made equal to 140°in order to allow observations being made of all the stars which passedthe meridian between the north pole and the southern horizon. Beingvery weakly constructed, the instrument was found to have warped whenplaced in the meridian. Flamsteed, however, was of opinion that withdue care it might be usefully employed notwithstanding this defect. Ac-cordingly, during the period included between the years 1683 and 1086he continued to determine the altitudes of the celestial bodies with it,and in the following year he constructed a small catalogue of stars to serveas points of reference in his future observations, founded upon the resultsobtained by the aid of this slight arc and the sextant f.

Having acquired possession of a small property by the death of hisfather in 1688, and seeing no prospect of being furnished with any instru-ments by the Government, Flamsteed now resolved to construct a strongmural arc at his own expense. He had already commenced operations forthis purpose when, his assistant Stafford dying in the month of May ofthe year above-mentioned, he engaged in his service the celebrated Abra-ham Sharp l- According to his plan of the instrument, the edge was to

* Baily’s Life of Flamsteed, p. 50.

+ This was a catalogue of 130 stars reduced to the year 1686, which he continued touse in computing from his observations the places of the moon and planets, even for sometime after he had commenced his observations with the mural arc in 1689.

X Abraham Sharp was born at Little Horton, near Bradford, in Yorkshire , about theyear 1651. After receiving the elements of a sound education at his native place, he wasput as an apprentice to a merchant in Manchester; but having no natural inclination to-wards commerce, he quitted his situation with his master’s consent, and subsequentlyopened a school in Liverpool, at the same time cultivating an acquaintance with mathe-matics and astronomy. Shortly afterwards he removed to London , where he becameknown to Flamsteed, who was then about to assume the superintendence of the RoyalObservatory of Greenwich . In 1688 he was appointed by Flamsteed to be his assistant, orrwhich occasion he constructed the famous mural arc with which that astronomer made themost valuable part of his observations. Finding that the fatigue of night observation did!not suit his weakly constitution, he retired to his native place, where he spent the re-mainder of his days, living upon the proceeds of a small patrimonial estate. Here heerected an observatory, which he fitted up with instruments all of his own construction.He also made his own telescopes, ground his own lenses, and constructed all sorts of toolsused by artificers, besides contriving and executing a great variety of ingenious pieces ofmechanism. Moreover he is the author of a mathematical work containing a collectionof most elaborate tables relating chiefly to logarithms. Although he lived in great re-tirement, he carried on an extensive epistolary correspondence with Newton, Halley, andall the most celebrated mathematicians of his time. In private life he was most exera-