περὶ ταῦτα—occupied in this. περὶ is much commoner in this sense than ἀμφί, which prep. occurs only twice in Thuc. and not at all in the Orators.
τότε—see c. 16.2. τότε is often used in referring back to what has been already mentioned. Cf. c. 31.1, 81, 2.
κατὰ πλοῦν—cf. καθ᾽ ὁδόν, κατὰ τὴν πορείαν, κατὰ τὴν στρατηγίαν.