Alexandria became the second stage for the civil wars of Caesar; here
the double drama of Antony and Cleopatra, then Caesar and Cleopatra
played before the skeptical populace. During the latter, Julius Caesar
supported Cleopatra against her brother Ptolemy XIV, and was besieged
by the latter's army and fleet in the Royal Precinct where was the
Museum. It was at this moment, in 48 B.C., that the most well-known
"burning of the Library" occurred, although many subsequent disasters
would later be hailed as the final destruction of the Library. The
legend apparently rose from Livy's account of the Alexandrine war, now
lost, but quoted by all subsequent scholars dealing with the topic,
including Seneca. Apparently, Livy stated that 400,000 rolls were
destroyed when, after Caesar set fire to the docks to block Ptolemy's
fleet, the flames consumed some nearby warehouses in which scrolls as
well as grain were being kept.[10] Scholars have debated hotly over
this conflagration since Roman times, disputing whether an actual
library was burned or whether these rolls had anything to do with the
Museum. Fraser is one of the most pursuasive advocates of the theory
that the Library, being in the Royal Precinct, was near enough to the
docks to be ignited. He suggests that the loss of 40,000 volumes also
explains why the Museum's library appears to gradually lost prestige to
the younger Serapeion.[11] Others assert that the fire was apocryphal,
or confused with later fires. Still others suggest that these books
were in fact copies waiting to be shipped to private collectors or
other libraries. Since, as Fraser points out during his discussion,
Didymus, Tryphon, and Theon researched in the Museum not long
afterwards, I lean towards the common theory that, if any books were
lost in this fire, copies of important works must have survived and the
library's collection not significantly damaged by the incident.
-- Ellen Brundige "The Decline of the Library and
Museum of Alexandria" December 10, 1991
http://www.digital-brilliance.com/kab/alex.htm