Having made these arrangements, he proceeded to lend aid at Chalkêdon, which the Athenians had already begun to attack. Their first operation after the victory, had been to sail to Perinthus and Selymbria, both of which had before revolted from Athens: the former, intimidated by the recent events, admitted them and rejoined itself to Athens; the latter resisted such a requisition, but ransomed itself from attack for the present, by the payment of a pecuniary fine. Alkibiadês then conducted them to Chalkêdon, opposite to Byzantium on the southernmost Asiatic border of the Bosphorus. To be masters of these two straits, the Bosphorus and the Hellespont, was a point of first-rate moment to Athens; first, because it enabled her to secure the arrival of the corn ships from the Euxine, for her own consumption; next, because she had it in her power to impose a tithe or due upon all the trading ships passing through, not unlike the dues imposed by the Danes at the Sound, even down to the present time. For the opposite reasons, of course, the importance of the position was equally great to the enemies of Athens. Until the spring of the preceding year, Athens had been undisputed mistress of both the straits. But the revolt of Abydos in the Hellespont (about April, 411 B.C.) and that of Byzantium with Chalkêdon in the Bosphorus (about June, 411 B.C.), had deprived her of this pre-eminence; and her supplies drained during the last few months could only have come through during those intervals when her fleets there stationed had the preponderance, so as to give them convoy. Accordingly, it is highly probable that her supplies of corn from the Euxine during the autumn of 411 B.C., had been comparatively restricted. Though Chalkêdon itself, assisted by Pharnabazus, still held out against Athens, Alkibiadês now took possession of Chrysopolis, its unfortified seaport, on the eastern coast of the Bosphorus opposite Byzantium. This place he fortified, established in it a squadron with a permanent garrison, and erected it into a regular tithing-port for levying toll on all vessels coming out of the Euxine.[177] The Athenians seem to have habitually levied this toll at Byzantium, until the revolt of that place, among their constant sources of revenue: it was now reëstablished under the auspices of Alkibiadês. In so far as it was levied on ships which brought their produce for sale and consumption at Athens, it was of course ultimately paid in the shape of increased price by Athenian citizens and metics. Thirty triremes under Theramenês, were left at Chrysopolis to enforce this levy, to convoy friendly merchantmen, and in other respects to serve as annoyance to the enemy. [177] See Demosthen. de Coronâ, c. 71; and Xenoph. Hellen. i, 1, 22. καὶ δεκατευτήριον κατεσκεύασαν ἐν αὐτῇ (Χρυσοπόλει), καὶ ~τὴν δεκάτην~ ἐξέλεγοντο τῶν ἐκ τοῦ Πόντου πλοίων: compare iv, 8, 27; and v, 1, 28; also Diodor. xiii, 64.