SEQUEL TO THE EPOCH OF HIPPARCHUS. 165 



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 

 parallactic instrument of Hipparchus 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 annexed 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 2d year of Antoninus, the 9th day of Pharmouthi, the sun 

 being near setting, the last division of Taurus being on the meridian 

 (that is, 5i equinoctial hours after noon), the moon was in 3 degrees 

 of Pisces, by her distance from the sun (which was 92 degrees, 8 

 minutes) ; and half an hour after, the sun being set, and the quarter 

 of Gemini on the meridian, Regulus appeared, by the other circle of 

 the astrolabe, 57^ degrees more forwards than the moon in longitude." 

 From these data the lono-itude of Regulus is calculated. 



From what has been said respecting the observations of the Alex- 

 andrian astronomers, it will have been seen that their instrumental 

 observations could not be depended on for any close accuracy. This 

 defect, after the general reception of the Hipparchian theory, operated 

 very unfavorably on the progress of the science. If they could have 

 traced the moon's place distinctly from day to day, they must soon 

 have discovered all the inequalities which were known to Tycho Brahe ; 

 and if they could have measured her parallax or her diameter with any 

 considerable accuracy, they must have obtained a confutation of the 

 epicycloidal form of her orbit. By the badness of their observations, 

 and the imperfect agreement of these with calculation, they not only 

 were prevented making such steps, but were led to receive the theory 

 with a servile assent and an indistinct apprehension, instead of that 

 rational conviction and intuitive clearness which would have given a 

 progressive impulse to their knowledge. 



16 Del. A. A. ii. 248. 



