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HISTORY OF PHYSICAL ASTRONOMY.

this purpose, were an iron sextant of seven feet radius, and two clocksgiven him by his kind patron, Sir Jonas Moore , and a quadrant and twotelescopes which he brought with him from Derby. The angles weremeasured with the sextant by the aid of a contrivance which Hooke hadsuggested in his “ Animadversions on the Machina Celestis of Hevelius.”A screw working upon the edge of the instrument (which was racked forthis purpose) imparted a slow motion to the moveable index, upon the endof which it was fixed, the angles being measured by counting the revolu-tions and parts of a revolution of the screw. The clocks were made byTompion, the most celebrated artist of the day. The pendulums werethirteen feet long, and made each a single vibration in two seconds oftime. The clocks required to be wound up only once in twelve months.The quadrant which Flamsteed brought with him from Derby had a radiusof three feet, and was employed by him in regulating his clocks by meansof the observed altitudes of the sun or stars *.

Flamsteed commenced his labours at the Royal Observatory of Green-wich on the 29th of October, 1676. The plan of observation at firstpursued by him was the same as that practised by Tycho Brahe and theearlier astronomers. The intermutual distances of the stars were deter-mined by observation, and their positions relative to two great circles inthe celestial sphere were deduced from these results by trigonometricalcalculation. Flamsteed had not long continued his observations, when hefound that Hooke’s contrivance for measuring angles with the sextantcould not be implicitly confided in. He, therefore, drew diagonals uponthe limb towards the end of the year 1677, and subsequently ex-pressed each angular distance determined with the instrument not only, ashitherto, in terms of the revolutions and parts of a revolution of thescrew, but also in degrees, minutes, and seconds. This practice wasfound by him to be very useful in checking any inaccurate reading of theobservations.

In order that he might be enabled to deduce useful results from hisobservations, Flamsteed was desirous of ascertaining the latitude of his ob-servatory, the place of the equinox among the stars, and other points ofessential importance; but these objects could not be effected by means ofthe sextant, which served only to determine the apparent positions of thecelestial bodies relative to each other. He perceived that for this purposea mural quadrant was necessary, and therefore as soon as he entered theobservatory he endeavoured to obtain one. Shortly afterwards, Hookewas charged by Sir Jonas Moore , to construct a mural quadrant of tenfeet for the observatory; but the instrument, when completed, was foundto be totally unmanageable. Flamsteed, therefore, now began to thinkhow he might take meridional altitudes with his sextant, and he so farsucceeded in his object as to be enabled, in 1677, to determine by its aidthe latitude of the observatory, which he found to be 51° 28' 10".

Flamsteed now proceeded to correct the elements of the solar theoryby means of his own observations. He devised on this occasion an inge-nious method for determining the absolute right ascension of the sun.

* Flamsteed states that he continued to use this quadrant till June, 1678, when heobtained a neater one from the Royal Society , which, however, he was compelled torestore to its owners in the month of October, of the following year. This circumstanceinduced him to construct a new quadrant of 50 inches radius, fitted with peculiar contri-vances, by means of which he was enabled to ascertain the time from observation withinthree seconds_( Daily’s Life of Flamsteed, p. 45.)