I'icart, aided, as it is said, by Auzout, first applied telescopic sights to graduated instruments. In his measure of the earth, executed in 1669 and 1670, he used a quadrant for his terrestrial angles. This he has described, with figures, in a special work, printed at the Louvre, which became so rare that the 'Academie Royale des Sciences' reprinted it in their Memoirs (vol. vii., part i., p. 133).. The quadrant was of 38 inches radius, with one telescope fixed in the direction of one radius, and the other moveable about the centre ; the are of the instrument was divided by transversals, and the angle read off by the index of the moveable telescope was equal to the angle subtended at the quadrant by the objects bisected by the cross-wires of the two telescopes. The instrument could be fixed on its stand with the plane vertical when used for altitudes, and by an additional piece, a genou, was moveable into any other plane, when it was wanted for surveying.
The whole turned on a vertical axis, like Tycho's azimuthal quadrants, but without an azimuthal circle. Quadrants like Picart's continued to be made by the French artists and used by their astronomers (with some improvements, of course) up to the latter end of the last century, when they were superseded by the repeating circle of Borda. The verification of the arc and of the graduation was performed in Tycho's manner, only employing well defined objects in the horizon instead of stars.
For a fixed observatory Plead and RoUnier recommended a large quadrant permanently fixed, that is, a copy of Tycho's mural quadrant, with the changes which telescopic sights required. Lemonnier, in the preface to his' Histoire Celeste,' says that La Hire had one erected in the Royal Observatory at Paris, in 1683, and that it was described in the first edition of La Hire's tables. This description he repeats at page xlii. of his work.
Flamsteed made his earlier observations at Greenwich with a sector, the plan of which may be understood by conceiving one of Tycho's sextants with telescopes, instead of plain sights, to be mounted on a polar axis. (‘ Historia Ceelestis; vol. iii., p. 103.) This instrument was designed for measuring the distances of stars from each other. But in pursuing his primary object, that of settling tho places of the fixed stars with accuracy, Flamsteed found that he required a meridian instrument. Some unlucky trials at constructing a quadrant were made by the person employed by the Royal Society, and Flamsteed finally constructed, at his own expense, and by Abraham Sharp's hands, tho mural arc with which he observed from 1689 to his death.*
(See the description and figure, € Historia Ccclestis,' vol. iii., p. 108.) This differed from other mural arcs chiefly in this, that it contained 140° or 150°, so that all stars were observable with it, from Polaris, below the pole, to the south horizon: The are was placed as nearly in the meridian as might be, and the errors in its plane detected by comparing the observed time of the sun's passage over the middle wire of the instrument, with the true time of his meridian passage, as deduced from corresponding altitudes with a quadrant. The pendulum clock, though as yet not a very perfect instrument, had by this time entirely done away with the neceseity of observing the mutual dis tances of the stars.
When Halley succeeded Flamsteed at Greenwich, the observatory appears to have been dismantled. Halley saw the great superiority of Roemer's transit over every other instrument for ascertaining right ascension, and accordingly introduced it; but ho seems not to have perceived the advantages which 'teenier's dreams raeridionabs pos sessed over any segment of a circle. In 1725 a mural quadrant was crested by Graham, which was superior to any previous Instrument of this construction ; it had however one grievous Imperfection : the radii being of iron and the arc of brass, every variation of temperature altered the value of the total arc. In 1750, this quadrant, which was subsequently known by the name of the iron or north quadrant, was removed to the other side of the pier, and the celebrated quadrant by Bird set up in its place. Of Bird's method of dividing we have given some account in the article GRADUATION. His which was a good deal based on this quadrant, Introduced similar instruments by himself or Ramsden into almost every observatory of note. Bird received 500/. from the commissioners of longitude for his Method of dividing Astronomical Instruments,' and the work was published by their order in 1767. We are not aware that a more perfect quadrant than the Greenwich brass or south quadrant was over constructed. It was with this instrument Bradley made his invaluable observations, which have been reduced with consummate skill by Besse'. (' Funda ments Astronoinke deducta ex Observationibus viri incotuparabilis James Bradley, autore F. W. Besse', Regiernontil 1818.) There is in this work a careful examination of the errors to which the two quadrants were liable.