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HISTORY OF PHYSICAL ASTRONOMY.

in observing corresponding altitudes of the sun and stars, for the purposeof regulating the clock, and adjusting the positions of the meridional in-struments.

Roemer also constructed an equatorial instrument, by means of whichhe was enabled to follow a celestial body throughout the whole of itscourse above the horizon, and also to measure the differences of the rightascensions and declinations of two stars situate very near to each other.Horrebow states that the object for which Roemer expressly constructedthis instrument, was to determine the parallaxes of the planets, by mea-suring their distances from the neighbouring stars at different altitudesabove the horizon. We have seen that in 1665 Hooke proposed to effectthe same object, by making observations of a similar kind in the meridianunder different latitudes.

There are few individuals, of ancient or modem times, who effectedimprovements in practical astronomy equal in importance to those due toRoemer. It is a remarkable fact that the Meridian Circle, the Altitudeand Azimuth Circle*, and the Transit Telescope, all of which wereinvented by this astronomer, form the capital instruments of the CentralObservatory of Russia , erected recently at Pulkowa, the most magnificentinstitution that has been established in ancient or modern times for thepurposes of astronomical observation.

Horrebow, who in 1710 succeeded Roemer as Astronomer Royal of Den-mark , has given a detailed account of the catastrophe which resulted inthe destruction of the astronomical observations of Roemer, as well as allthose made by himself. He had been engaged at his own house on theevening of October 20, 1728, in writing a commentary upon a fragmentof Roemer’s on the Annual Parallax of the Terrestrial Orbit, which hehad a short time previously discovered among the manuscripts of thatastronomer, when his attention was aroused by a confused noise of drumsand bells, announcing that a serious conflagration had broken out in thecity f. As the place where the fire was raging, -was at a considerable dis-tance from his own house, he was for some time in hopes that it would beextinguished without occasioning him any inconvenience. The wind,however, having unfortunately shifted about eight o’clock in the evening,the fire now began to approach his own dwelling-house. Presently frag-ments of burning material fell into his court, while other portions, alight-ing on the roof of his house, threatened it with instant danger. Therewas now no time to be lost. His maid-servants were hastily conductedout of his bouse, along with eight children, four of whom were in a stateof nudity, while the nurse carried in her arms an infant which had beenonly recently born. His wife and his eldest son, a youth of sixteen yearsof age, remained with him, and, with other friends, assisted in transport-ing his effects to the Hall of the Academy. This new place of refuge

* The use of the Vertical Circle of Pulkowa is indeed different from that of Roemer’sinstrument, being employed mainly in observing the celestial bodies a few minutes beforeand after their culmination.

+ The mode in which poor Horrebow endeavours to bring the dreadful catastrophe be-fore the mind of the reader is quite dramatic. He supposes himself to be engaged inwriting his commentary, and to be interrupted in the midst of his labours (as was reallythe case) in the following manner :—“ Namque ex observationibus ab anno 1692 usque ad1702 ipsum beatum authorem parallaxia orbis annui deduxisse ex tota dissertatione, im-primis, § 160, clarum est. Sed quid audio ! Tumultus hominum in plateis cursitantiumet clamantium ! Sonitus fisttdarum, tympanorum, campanarum, Sfc. Eheu ! incendiumultra modum vires et increments sumens. Die 20 Octob. 1728, hor. 7, vespert. ( BasisAstronomies, p. 70.)