SEQUEL TO THE EPOCH OF HIPPARCHUS. 207 



days which it contains; but the day can be sub- 

 divided into hours only by artificial means; and 

 the mechanical skill of the ancients did not enable 

 them to attain any considerable accuracy in the 

 measure of such portions of time; though clep- 

 sydras and similar instruments were used by astro- 

 nomers. The equality of days could only be proved, 

 therefore, by the consequences of such a suppo- 

 sition ; and in this manner it appears to have been 

 assumed, as the fact really is, that the apparent 

 revolution of the stars is accurately uniform, never 

 becoming either quicker or slower. It followed as 

 a consequence of this, that the solar days (or rather 

 the nycthemers, compounded of a night and a day,) 

 would be unequal, in consequence of the sun's 

 unequal motion, thus giving rise to what we now 

 call the Equation of Time, the interval by which 

 the time, as marked on a dial, is before or after 

 the time, as indicated by the accurate time-pieces 

 which modern skill can produce. This inequality 

 was fully taken account of by the ancient astro- 

 nomers ; and they thus in fact assumed the equality 

 of the sidereal days. 



Sect. 2. Researches which did not verify the 

 Theory. 



SOME of the researches of Hipparchus and his fol- 

 lowers fell upon the weak parts of his theory; 

 and if the observations had been sufficiently exact, 

 must have led to its being corrected or rejected. 



