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Chapter 7
Plate 7.3. Two pages from Yamaguchi’s diary, showing several problems, including the one given as problem 10 in this chapter. (Agano City.)
22nd of July, 1820 to 21st of August, 1820 Y1 22nd of July, 1820: “Many friends came to see me off in Edo for my long travels and left me send- off haiku . . . After 10 ri [40 km], I arrived in Fuchu and visited the Roku shrine. At night, I stayed at a farmer Yohachi’s house.” 26th of July: “I passed the castle town1 of Takasaki and I remembered that a mathematician Ono Eijyu lived there.”2
Y2 27th of July: “I have visited the Hakuunsan Myoujin shrine and walked across a river nearby since there was little water in the river. After passing 1 Although “castle towns” had their origins as military strongholds, as discussed in chapter 1 the Tokugawa shogunate limited the local warlords to one castle per domain. With little fighting during the entire Edo period, the castle towns became administrative centers and the castle took on the aspect of “city hall.” “Castle town” thus has a connotation closer to “provincial capital” than to “fortified town.” 2 Mathematician Ono Eijyu (1763–1831), a student of Fujita Sadasuke’s school (chapter 3) had been training many students to be mathematicians and sometimes helped geographer Ino¯ Tadataka (1745–1818) to produce his map of Japan.