HISTORY OF PHYSICAL ASTRONOMY.
463
Roemer, in his views of practical astronomy, seems to have been farin advance of the age in which he lived. He makes some admirableremarks on this subject, in a letter dated December 15, 1700, addressedby him to Leibnitz , who had solicited his opinion upon the most suitableplan of an observatory. “ I differ very widely,” says he, “ from thosewho have hitherto decked out observatories more for show than for use,accommodating the instruments to the buildings rather than the buildingsto the instruments.”* * * § The illustrious Dane, while writing these lines,had doubtless a vivid recollection of the magnificent “ Observatoire deparade f ” on the banks of the Seine , and of the expedients which Picardand himself were compelled to adopt, while endeavouring to fix a quadrantin the plane of the meridian, in the year 1678 j. The superiority of cir-cular instruments, when compared with the quadrant and sextant, did notescape his sagacity: “Ido not concur with others,” says he, “ in then-opinion respecting the construction of instruments, since I consider thatthe use of the quadrant and sextant should be altogether abandoned,for I would place more confidence in a circle of four feet, than I would ina quadrant of ten feet.”§ He then proceeds to describe the plan of anobservatory, which he had long desired for himself, and the characterof the instruments which he designed to employ in it. In both theserespects his wishes were fully realised a few years afterwards, as ap-pears from the following description of his Rural Observatory, given byHorrebow :—
“ The ‘ Observatorium Tusculaneum ’ of Roemer was built in an opensituation upon a gentle eminence, a little to the west of the ancientobservatory. It was 17 Danish feet in length, the walls were 5| feethigh outside and 6| feet inside. The observations in the meridianwere made with an instrument termed the Rota Meridiana, or Meridiancircle. It consisted of a circle of 5| feet in diameter, revolving in theplane of the meridian at one of the extremities of a horizontal axis ofequal length. A telescope 5 feet long was attached to the eastern faceof the circle. The intrument rested upon two solid supports of fir-wood,fitted with bars of metal into which the extremities of the axis wereinserted. In order to prevent flexure, the axis was composed of twohollow cones of iron attached at their bases. The focus of the telescopewas traversed by seven vertical and three horizontal threads. The divi-sions of the instrument w T ere read by means of two microscopes fixed op-posite the face of the circle at a distance of 10° from each other. Themode of reading off was similar to that practised in the case of the instru-ment already referred to. In order that the instrument might commandthe whole of the meridian from the northern to the southern horizon, acontinuous fissure about 4 inches broad, was formed in the walls and roofof the building, the parts being united by iron bars only in those placeswhere no stars were apparent.
Roemer did not fail to verify his instrument before commencing his
* Miscellanea Beroliniensia, tome iii,, p. 277. f M. Arago, Annuaire, 1845.
f Horrebow, in his Basis Astronomies , cites a statement of Roemer’s to the effect thathe, in conjunction with Picard, fixed a mural quadrant in the plane of the meridian, atthe Royal Observatory of Paris, in the year 1678. This seems to be contradicted by La
Hire, who asserts that the mural quadrant was adjusted permanently in its position only inthe year 1683. It is probable that the expedient employed by Picard and Roemer wasmerely of a temporary nature.
§ Miscel. Berol., tome iii., p. 277.