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HISTOEY OF PHYSICAL ASTRONOMY.

be racked so as to measure the angles by Hooke’s method, while at thesame time, as in the case of the sextant, diagonals were to be drawn uponthe limb. Sharp, who was at once an ingenious mathematician, an expertcalculator, and a skilful mechanic, racked the edge of the limb, preparedthe index, and fixed the arc on the wall. Afterwards he divided and en-graved the limb “ so exquisitely,” says Flamsteed, “ as to excite the ad-miration of every beholder.” The instrument was not completely readyfor use till the month of October, 1089'. Fourteen months were occupiedin its erection, and the total cost to Flamsteed was no less than one hun-dred and twenty pounds, not one farthing of which was ever refunded tohim by the Government.

The mural arc erected by Flamsteed on this occasion was made equal to140°, as in the case of the former instrument of the same description.Its radius was 79 inches in length. The limb was, in the first instance,divided to every 5', and was then subdivided into smaller parts bymeans of diagonals. For this purpose the fiducial edge of the moveableindex, included between the outer and inner arcs of the instrument, wasdivided into five parts so as to indicate minutes *, and each minute wassubdivided into six equal parts, each equal to 10", which again might bebisected by estimation so as to obtain a final subdivision to every 5".

Before commencing his observations with the mural arc, Flamsteedtook the precaution of ascertaining whether the zero point of division hadundergone any alteration with respect to the zenith, during the intervalthat had elapsed since it was marked upon the limb. This was effected byobserving certain stars near the zenith, first with the instrument itselfattached to the eastern face of the meridian wall, and then observingthe same stars with the telescopic index of the instrument transported tothe western face of the wall. Half the difference of the zenith distancesof each star determined in this manner, gave the corresponding error ofcollimation of the instrument, and the mean of all the results gave themean error of collimation. From observations of this kind, Flamsteedfound that the zenith distances of those stars which passed the meridiantowards the north, were all too great as determined by the instrument.The error on the 11th of September, 1689, amounted to 45". In thecourse of his subsequent observations, he found that the error increasedslowly in the same direction every year, a circumstance which he at-tributed to the gradual sinking of the southern extremity of the wall towhich the instrument was attached. He did not fail, therefore, to deter-mine from time to time, by means of suitable observations, the amount ofdeviation arising from this cause. He remarked that the stars in the feetof the constellation Gemini afforded peculiar advantages for this purpose;for being situate near the solstitial colure, they were liable to change veryslowly from the retrograde motion of the equinoctial points, and, being at

plary. He died on the 18th of July, 1742, in the ninety-first year of his age. Thedelicacy of Sharp’s hand for manipulative operations was exquisite. He is generallyadmitted to be the first person who cut divisions on astronomical instruments with anypretension to accuracy. He is also the first of those remarkable men by the aid of whosemechanical talents the Astronomers of the Royal Observatory of Greenwich were enabledto execute the series of observations which have won for that establishment its unrivalledreputation.

* The intervals in this case, being the results of a strict mathematical process, were notexactly equal, but the difference between two consecutive divisions could scarcely be con-sidered sensible.