Complex sentence: By the time it passed to the British Museum in 1827 it had grown to 65,000 printed volumes, as well as 17,500 pamphlets and 446 manuscripts; the manuscripts mainly date from the reign of George III and few are of particular note.
Split into simpler sentences: By the time it passed to the British Museum in 1827 it had grown to 65,000 printed books. In addition there were 17,500 pamphlets and 446 manuscripts; the manuscripts mainly date from the reign of George III and few are of particular note.