Dec. 6. 1877] 



NATURE 



107 



planispheres. He was, in 1679, sent by the Academy to 

 London, to examine the English determination of the 

 length of the second pendulum. He took part in the 

 levellings necessary for conducting water to Versailles, 



Fjg. 2. — Mura! Circle, U.S. Naval Observatory. 



which gave him occasion to write several interesting 

 papers. He made many observations in Paris. Already 

 in 1671 he had taken part in the observat ons of the 

 altitude of Mars, which was simultaneously 

 observed by Richer in Cayenne, for the deter- 

 mination of the parallax of the sun. His fame 

 increased so much that he was made tutor to 

 the Dauphin, and in 1681 Christian V, called 

 him to Denmark as Astronomer- Royal. His 

 great technical knowledge made him useful to 

 that country in many ways, and we see him in 

 succession as Professor of Mathematics, Mayor 

 of Copenhagen, Master of the Mint, Prefect of 

 the Police, Privy- Councillor, and one of the 

 Judges of the Supreme Court, in all of which 

 capacities he left behind a lasting fame. He 

 was one of the first who recommended Pro- 

 testants to adopt the calendar as reformed by 

 Gregory XIII. He had also to make a journey 

 in 1687 to acquaint himself with the latest 

 progress in naval architecture and pyro- 

 technics. We cannot fail to respect his per- 

 severance when we hear that, notwithstanding 

 so many different occupations, he left behind 

 about as many observations as Tycho Brahe 

 himself. But these were mostly all lost by the 

 great fire which devastated the town in 1728. 



Romer found in Copenhagen the old obser- 

 vatory of Longomontanus on " the round 

 tower" almost devoid of instruments, and it 

 was first in 1690 that two were placed there. 

 One of these was not unlike a modern equa- 

 torial, and intended for extra-meridian obser- 

 vations ; but it was generally clamped in the 

 meridian, and used as a transit circle. The 

 other was a vertical circle for taking corre- 

 sponding altitudes, a method much used by Picard. The 

 position of these instruments on the top of the tower (over 



under the open air, rendered their use^ however, so in- 

 convenient to the observer, that Romer about the same 

 time arranged an observatory in one of the windows of 

 his dwelling house. Here was placed the transit instru- 

 ment which Romer invented, but it was greatly inferior to 

 the instrument he afterwards constructed, Tne telescope 

 was not fixed in the middle of the horizontal axis as in 

 modern instruments, but near one end. The axis, which 

 rested on iron supports in the wall, was a long and thin 

 iron bar, furnished with a counterpoise acting in the 

 middle, to preveat flexure. The tube itself was cone- 

 shaped for the same reason. In the focus were drawn a 

 horizontal and a number of vertical wires. The interval 

 between the three he generally used was thirty-four 

 seconds in the equator, and the time was noted to half 

 seconds. The held was illuminated by means of a 

 polished ring placed outside of the object-glass. The 

 circle was not movable with the telescope but fixed to the 

 wall, and the telescope carried with it a microscope fixed 

 upon an arm for reading the declinations. The arc was 

 divided to ten minutes and in the microscope were eleven 

 wires, each one minute distant from its neighbour. 

 The minutes were read thus and could be sub- 

 divided by estimates to about four seconds. The 

 instrument being placed in a window Romer could only 

 observe the stars of between twenty-eight south and forty 

 degrees north declination, and the arc was therefore not a 

 whole circle but merely about seventy degrees. The error 

 of collimation was corrected by reversion. The azimu h 

 was ascertained by comparing the observed error of the 

 clock with that determined by corresponding altitudes. 

 It was at this observatory that Romer tried from obser- 

 vations of the right ascensions ©f two bright stars on 

 opposite sides of the sky, to determine the sum of their 

 parallaxes. 



But these arrangements did not long satisfy Romer, and 

 in 1704 he b^iilt, at his own cost, the " Observat orium 

 Tusculaneum," seventeen feet long and broad, near the. 

 village VridlQsemagle, between Copenhagen and Roes- 



FiG. 3. — Meridian Circle, U.S Naval Observatory. 



kilde. The principal instrument of this observatory was 



a meridian circle, and the stars were observe! through a 



100 feet high), where the observations had to be made 1 very narrow opening in the ceiling and the walls running 



