eclipses was in use. Here, again, patient observation would easily
lead to the detection of the eighteen-year cycle known to the
Chaldeans as the _Saros_. It consists of 235 lunations, and in
that time the pole of the moon's orbit revolves just once round the
pole of the ecliptic, and for this reason the eclipses in one cycle
are repeated with very slight modification in the next cycle, and so
on for many centuries.

It may be that the neglect of their duties by Hi and Ho, and their
punishment, influenced Chinese astronomy; or that the succeeding
records have not been available to later scholars; but the fact
remains that--although at long intervals observations were made of
eclipses, comets, and falling stars, and of the position of the
solstices, and of the obliquity of the ecliptic--records become rare,
until 776 B.C., when eclipses began to be recorded once more with some
approach to continuity. Shortly afterwards notices of comets were
added. Biot gave a list of these, and Mr. John Williams, in 1871,
published _Observations of Comets from 611 B.C. to 1640 A.D.,
Extracted from the Chinese Annals_.

With regard to those centuries concerning which we have no
astronomical Chinese records, it is fair to state that it is recorded
that some centuries before the Christian era, in the reign of
Tsin-Chi-Hoang, all the classical and scientific books that could be
found were ordered to be destroyed. If true, our loss therefrom is as
great as from the burning of the Alexandrian library by the Caliph
Omar. He burnt all the books because he held that they must be either
consistent or inconsistent with the Koran, and in the one case they
were superfluous, in the other case objectionable.