THEORY OF THE EARTH. 217 



" The precession of 50" yearly has been supposed, 

 this being the result of the comparison of the 

 catalogue of Hipparchus with the modern cata- 

 logues. We have thus the advantage of round 

 numbers, and a general accuracy that may be re- 

 lied upon. The entire period is thus 25,920 

 years; the semiperiod, 12,960 years; the quar- 

 ter period, 6480 years ; the twelfth, or a sign, 

 2160 years. 



" It is to be remarked, that the constellations 

 leave empty spaces between them, and that some- 

 times they encroach upon each other. Thus, be- 

 tween the last star of Scorpio, and the first of 

 Sagittarius, there is an interval of 6 degrees. 

 On the other hand, the last of Capricorn is more 

 advanced by 14 in longitude, than the first of 

 Aquarius. Hence, even independently of the in- 

 equality of the sun's motion, the constellations 

 would afford a very unequal and very erroneous 

 measure of the year and its months. The signs 

 of 30 furnish a more convenient and less defec- 

 tive one. But the signs are merely a geometri- 

 cal conception ; they can neither be distinguished 

 nor observed ; and they are continually changing 

 place from the retrogradation of the equinoxial 

 point. 



" We have at all times been able to determine, 

 in a rough manner, the equinoxes and solstices ; 



