484

HISTORY OF PHYSICAL ASTRONOMY.

divided to every 5'; then the intervals between two consecutive divisionswere subdivided by the vernier to every 30", and these were finally sub-divided by the micrometer screw to every 1". By the outer arc thequadrant was divided into ninety-six parts; then these were subdividedby the vernier into sixteen smaller parts, each equal to 13".183G ; andthese were finally subdivided by the micrometer screw to every ]". TheGovernment at the same time furnished the observatory with a new rtransit telescope by Bird , eight feet long, besides an excellent clock byShelton, and several other instruments of minor importance. They alsopurchased of Bradley the famous zenith sector with which he discoveredthe phenomena of aberration and nutation, and appropriated it to the useof the observatory. This noble instrument w'as designed by Bradley tobe henceforward employed in determining the errors of collimation of thequadrants, by making observations with it when its face was turned alter-nately east and west.

Bradley being now in possession of instruments of unequalled perfec-tion, proceeded to execute a series of preliminary observations for thepurpose of ascertaining with greater accuracy the latitude of his observa-tory, the place of the equinox, the quantity and laws of refraction, andother fundamental points of astronomy. In pursuance of this design thebrass quadrant was turned towards the north, so as to be employed inmaking observations of circumpolar stars. These observations were pro-secuted from August 10, 1750, to July 31, 1753. From them Bradleydeduced the latitude of the observatory to be 51° 98' 38|". He alsosucceeded by means of them in constructing the elegant rule, so longused by astronomers, for finding the quantity of refraction correspondingto any assigned zenith distance, and any observed readings of the baro-meter and thermometer. He determined the absolute right ascensionsof a few of the principal stars, by means of observations made near theequinoxes, according to the method of Flamsteed.

As soon as Bradley had established these fundamental points he re-moved the brass quadrant from the western face of the meridian wall,and permanently attached it to the eastern face, where it was afterwardsemployed in observing the stars that passed the meridian to the south ofthe zenith. At the same time the iron quadrant was removed from theeastern face of the wall, and, after being re-divided by Bird , was attachedto the western face, for the purpose of making observations with thetelescope turned towards the north. Bradley now commenced the seriesof admirable observations wdiich have formed the groundwork of so muchvaluable research to future enquirers, and which would have assured tohim an immortal reputation, even independently of those great discoverieswith which his name is inseparably associated. The sun, moon, andprincipal stars, and the planets when situate in favourable positions, wereregularly observed with the transit instrument and the mural quadrants.Moreover, a multitude of small stars, chiefly those of Flamsteed’s cata-logue, were included in the plan of observation. From the year 1750may be dated the commencement of a series of observations which inpoint of accuracy may bear a comparison with those of modern times.Henceforward the records of Greenwich Observatory embody a collectionof materials, which have almost exclusively formed the groundwork ofevery investigation undertaken in modern times, for the purpose of im-proving the solar, lunar, or planetary tables.

Bradley, aided by only one assistant, continued with indefatigable assi-