On the 16th, Mahomet, probably because he had learnt of the landing of Trevisano’s men from the fleet, ordered his ships at the Double Columns to make another attack upon the boom. One would have expected that the seventy or eighty ships that were in the Inner Horn would have co-operated in this attack but they did not move. Neither Turk nor Genoese cared to risk open war with the other. The Turkish fleet came down the Bosporus, and the Greek and Venetian ships prepared to receive them. As the Turkish ships came up to the attack, Diedo brought his vessels from the shelter of the walls of Galata to the boom. Thereupon the Turks retired, and using their oars returned to the Columns. A similar incident occurred on the 17th, but the Turks, again finding that the ships at the boom were prepared for a fight, went back. [Sidenote: Renewed attempt on May 21.] Mahomet, however unwilling to break with the Genoese, was not content to have communication between the two divisions of his fleet interrupted. Accordingly, once more he renewed his attempt to destroy the boom. Barbaro appears to have been on one of the ships defending it. On May 21 at two hours before daylight, the whole fleet moved out from the Double Columns and with great noise of drums and trumpets came down the Bosporus. All on board the Christian vessels were greatly alarmed, but dispositions for the defence were taken, and, as it was feared that contemporaneously a general attack upon the city was about to be made, the alarm bells rang out and every one took his allotted station either on shore or on the ships. Once more the Turks decided that it was hopeless to attempt the destruction of the boom, and therefore returned to their moorings. It is impossible to say whether the Turks really believed that they might destroy it or whether the three attempts just mentioned were merely feints to tire out the besieged and alarm them by a display of overwhelming force. It is certain, however, that the Venetian and Greek sailors were always ready to resist, and that, after this attempt on May 21, Mahomet’s fleet made no further attempt to force its way into the harbour. [Sidenote: Attempts to undermine the walls.]