478

HISTORY OF PHYSICAL ASTRONOMY.

were all his own, the Government not having even defrayed the expenseof occasionally repairing them. We need not wonder, therefore, thatwhen Halley entered the observatory he found it completely stripped ofeverything available for the prosecution of celestial observations. Themural arc, the clocks, the telescopes, and, in short, every instrument inthe observatory, had been carried away by Flamsteed’s executors imme-diately after his death. In consequence of this circumstance, some timeelapsed before Halley was enabled to enter upon a regular course ofobservation. In 1721 he procured a transit instrument, with which hecommenced observations of the passages of the celestial bodies over themeridian. This instrument was five feet six inches long, and is stated byLalande to have been constructed by Hooke. Halley’s first observationwith it is dated October 1, 1721. In 1726, a mural quadrant, for theobservation of zenith distances as well as differences of right ascension,was at length erected. This instrument was constructed by the cele-brated artist Graham*. It was made of iron, and had a radius of eightfeet. The limb was composed of two arcs, upon each of which a differentmode of division was executed. The inner arc was divided to every 5',according to the ordinary sexagesimal scale, and was afterwards sub-divided, by means of a vernier, to every 30", which again might be sub-divided, by estimation, to one-fourth of this quantity, or r t\". The outerarc was divided, first, into ninety-six parts, and then each of these intosixteen smaller parts, each of which again was subdivided, by means of avernier, to every 13", and each of these parts might finally be subdivided,by estimation, to 5" or 6". In making observations with the instrument, bothmodes of reading off were employed, so that the one might serve as a checkupon the other; but in cases of discordance, greater reliance was usuallyplaced on the indications of the outer arc. Halley commenced his obser-vations with this instrument on the 20th of October, 1725. Hencefor-ward he continued to observe not only the transits of the celestial bodiesover the meridian, but also their zenith distances. It does not appear,however, that he adopted such an extensive plan of observation as thatpursued by his predecessor, Flamsteed. His main object was to procurematerials for the improvement of the lunar and planetary tables; and, forthis purpose, he deemed that it would be unnecessary to observe any otherstars than those that were situate within the limits of the zodiac. Hislabours were more especially directed to the observation of the moon.He was stimulated to this object by the large reward which the Govern-ment had offered relative to the longitude, being convinced that themethod of lunar distances was better adapted than any other to thesolution of that important problem. This subject seems to have so farengrossed his attention as to have caused him in some degree to neglectthe ordinary duties of his office. It has been already mentioned that the

* George Graham was born at. Gratwick, in Cumberland, in the year 1675. He wasnot only the most skilful artist of his time, but was also distinguished by a profoundacquaintance with various subjects of natural philosophy. Besides the mural quadrantalluded to in the text, he constructed the famous zenith sector with which Bradley dis-covered the phenomena of aberration and nutation. It was also with a zenith sectorexecuted by the same artist that the French academicians, who measured the length of anarc of the meridian in Lapland , determined the latitudes of their stations. Graham con-tributed various papers to the Philosophical Transactions of the Royal Society , of whichhe was a member. One of these related to the mercurial compensation pendulum, theinvention of which is due to him. He died in 1751, aged 76 years.