HISTORY OF PHYSICAL ASTRONOMY.

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instruments, and carefully to adjust their positions. Halley had confinedhimself to the use of the mural quadrant soon after its erection in 1735 *.Bradley, however, resolved to employ both the quadrant and the transitinstrument in his observations. The former of these instruments wasrepaired by Graham, and the latter by Sisson, several important improve-ments being at the same time effected in their construction. Bradley’sfirst observation witli the quadrant was made on the 15th of June, 1743.His earliest transit observation is dated the SOtli of July in the sameyear.

The mode in which Graham’s iron quadrant was divided has beenalready mentioned. Halley generally adopted the nearest reading of thevernier without regard to the fractional parts, but occasionally he markedthe half of the interval between two divisions of the vernier. The errorof his observations might, consequently, amount from this cause to 5"or 0". Bradley pursued a similar plan of observation till July 18, 1745,when, having applied a new micrometer screw to his instrument, thethreads of which were much finer than those of the old one, he employedit to measure the fractional parts of the angles. One revolution of thescrew altered the angle upon the instrument 53"; and, consequently, thefifty-third part of a revolution was equivalent to 1". By this applicationof the micrometer, Bradley was now enabled to make observations toseconds with as great accuracy as he had formerly done to divisions ofthe vernier f.

The iron quadrant of Graham continued to be attached to the easternface of the wall erected for its support in 1735, and was employed bothby Halley and Bradley in observing the celestial bodies which passed themeridian to the south of the zenith. Bradley, however, was desirous ofextending the plan of his observations, and with a view to this object hepresented a memorial to the Government in the year 1749, solicitinganother quadrant, by means of which the stars that passed the meridianto the north of the zenith might be observed. The Government, with apromptitude hitherto unusual, at once acceded to his views, and in thefollowing year the observatory was furnished with a magnificent brassquadrant of eight feet radius constructed by Bird ];, who now took theplace of Graham, as the most skilful divider of instruments in his day.As in the case of the iron quadrant, there were two different modes ofdivision executed upon the limb. By the inner arc the quadrant was

* This course was imperatively prescribed by the necessities of his situation, for therewas only one vertical wire in his transit, and he had no assistant by whose co-operationhe might be enabled to make simultaneous observations with two instruments. As soonas Bradley was appointed Astronomer Royal he succeeded in procuring a regular assistant,who was paid by the Government. His first assistant was his nephew, John Bradley,who, according to Lalande, resided in that capacity at the Royal Observatory for a periodof fifteen years.

f Rigaud’s Miscellaneous Works, A c., of Bradley, p. lv.

J John Bird was born in 1709; he was brought up a cloth-weaver in the county ofDurham . It is said that his attention was first drawn to the dividing of instruments, byobserving the imperfect graduation of a clock dial. Having acquired considerableexpertness in his new vocation, he proceeded to London , where he was employed byGraham and other artists of the day. After residing in London for a few years, he com-menced business on his own account, and soon attained an unrivalled reputation as adivider of instruments. In 1767 he received .£500 from the Government, on conditionthat he should divulge his method of graduation. This he did in a work which he pub-lished on the subject. ( The Art of Dividing Instruments, London , 1767.) He died onthe 31st of March, 1776.

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