SEQUEL TO THE EPOCH OF HIPPARCHUS. 217 



regard to the sun or a star, were thus deter- 

 mined. 



The astrolabe continued long in use, but not so 

 long as the quadrant described by Ptolemy ; this 

 in a larger form, is the mural quadrant, which 

 has been used up to the most recent times. 



It may be considered surprising 15 , that Hip- 

 parchus, after having observed, for some time, 

 right ascensions and declinations, quitted equatorial 

 armils for the astrolabe, which immediately refers 

 the stars to the ecliptic. He probably did this 

 because, after the discovery of precession, he found 

 the latitudes of the stars Constant, and wanted to 

 ascertain their motion in longitude. 



To the above instruments, may be added the 

 dioptra and the parattactic instrument of Hippar- 

 chus, and Ptolemy. In the latter, the distance of 

 a star from the zenith was observed by looking 

 through two sights fixed in a rule, this being an- 

 nexed to another rule, which was kept in a vertical 

 position by a plumb-line; and the angle between 

 the two rules was measured. 



The following example of an observation, taken 

 from Ptolemy, may serve to show the form in 

 which the results of the instruments, just described, 

 were usually stated 16 . 



" In the 2nd year of Antoninus, the 9th day of 

 Pharmouthi, the sun being near setting, the last 

 division of Taurus being on the meridian (that is, 

 16 Del. A. A. 181. " Del. A. A. ii. 248. 



