HISTORY OF PHYSICAL ASTRONOMY.

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the index from the nearest point of division was ascertained by means ofa micrometer screw. This mode of reading off angles was first proposedby the Due de Ohaulnes, in a small work which he published at Paris inthe year 1768. Ramsden did not fail to see the advantages of it, andaccordingly he applied it to the altitude and azimuth circle which he con-structed for Piazzi.

Towards the close of his career at Greenwich , Maskelyne began tosuspect, from certain irregularities in the declinations of the stars, asdetermined with Bird ’s mural quadrant, that the instrument had becomedefective from long use. This fact having been established beyond doubtby evidence derived from other sources, he represented to the Govern-ment the expediency of replacing it by a circular instrument. Thisrequest was acceded to, and a mural circle of six feet in diameter wasconstructed for the Royal Observatory by Troughton*, but it was notactually finished when Maskelyne died.

John Pond, the successor of Maskelyne at the Royal Observatory ofGreenwich, was born in London in the year 1767. At the age of sixteenhe was placed at Trinity College, Cambridge. While engaged in prose-cuting his studies there, he displayed an interesting proof of his inclina-tion towards those pursuits which subsequently formed the occupation ofhis life, by uniting with two of his fellow-students in soliciting Prof. Yinceto give a course of lectures on practical astronomy. After leaving Cam-bridge, he proceeded to the Continent , where he continued to travel forsome time. Upon his return to England he established his residence atWestbury, in Somersetshire , and devoted his whole attention to practicalastronomy. Having procured an altitude and azimuth circle of three feetdiameter, constructed by Troughton, he commenced a series of observa-tions with it, and so successfully did he prosecute his labours, that in 1806he was enabled to demonstrate by irrefragable evidence founded upon theresults, that the brass quadrant of Greenwich had undergone a change ofform since its erection in Bradley’s timef. His earliest observation atGreenwich is dated the 11th of January, 1811. In the month of June ofthe same year the mural circle which Troughton had been charged toconstruct for the Observatory was ready for use. This magnificent instru-ment demanded a method of observation totally different from thathitherto pursued with the quadrants. In his mode of using it Pondshewed that he possessed talents adequate to the occasion. The observa-tions of a celestial object were now made, not by determining the distancefrom the zenith agreeably to the practice hitherto pursued with the quad-rants, but by referring the object to the pole as the zero of measurement,

* Edward Troughton was born m 1753 in the parish of Corney in Cumberlandshire.He was brought up to farming pursuits till he was seventeen years of age, but he thenproceeded to London, and acquired instruction as an artist under his brother John . In1782, the two brothers commenced business on their own account, and by their industryand skill soon attained a prosperous condition. Upon the death of his brother, Edwardcontinued to prosecute the business of an artist for many years, and ultimately acquired aEuropean reputation by his excellence as a constructor of astronomical instruments. In1830, he received an honorary gold medal from the King of Denmark. He died on the12th of June, 1835. A marble bust of this distinguished artist adorns the Observatoryof Greenwich. He was the last of a succession of English artists, each of whom waswithout a rival in his day. It is somewhat singular that Sharp, Graham, Bird , Ramsden,and Troughton should have succeeded each other at almost equal intervals; and stillmore so, that they should all have been born in a tract of country comprehended withinthe limits of three adjacent counties of the north of England.

+ Phil. Trans., 1806, p. 420, et seq.