88 THE HISTORY OF SCIENCE 



great mural quadrant at the observatory of Urani- 

 borg. He called it with characteristic vanity the 

 Tichonic quadrant. It consisted of a graduated arc 

 of solid polished brass five inches broad, two inches 

 thick, and with a radius of about six and three quar- 

 ters feet. Each degree was divided into minutes, and 

 each minute into six parts. Each of these parts was 

 then subdivided into ten seconds, which were indi- 

 cated by dots arranged in transverse oblique lines on 

 the width of brass. 



The arc was attached in the observation room to 

 a wall running exactly north, and so secured with 

 screws (firmissimis cochleis) that no force could 

 move it. With its concavity toward the southern sky 

 it was closely comparable, though reverse, to the 

 celestial meridian throughout its length from horizon 

 to zenith. The south wall, above the point where the 

 radii of the quadrant met, was pierced by a cylinder 

 of gilded brass placed in a rectangular opening, which 

 could be opened or closed from the outside. The ob- 

 servation was made through one of two sights that 

 were attached to the graduated arc and could be 

 moved from point to point on it. In the sights were 

 parallel slits, right, left, upper, lower. If the alti- 

 tude and the transit through the meridian were to 

 be taken at the same time the four directions were to 

 be followed. It was the practice for the student mak- 

 ing the observation to read off the number of degrees, 

 minutes, etc., of the angle at which the altitude or 

 transit was observed, so that it might be recorded by 

 a second student. A third took the time from two 

 clock dials when the observer gave the signal, and the 

 exact moment of observation was also recorded by 



