Nabonetus and a part of the fugitives were thus cut off from Babylon, he could only retire southwards beyond the Euphrates into the city nearest Babylon, _i.e._ into Borsippa, to seek protection at the great temple of Nebo (I. 291), the god whose name he bore.[75] The command in Babylon then devolved on his eldest son Bil-sar-ussur (p. 67). It follows from the narrative of Berosus that Cyrus quickly followed up the defeated army of the Babylonians, that a part of the Persians, treading on the heels of the fugitives, crossed the Euphrates below the city, to invest Borsippa and the metropolis on the western side. Berosus has told us that Cyrus marched against Babylon with a great force. His army must indeed have been strong enough to enclose the second circuit of the city, 35 or 40 miles (III. 372), to meet the attack of the whole force of the besieged on both sides of the river, and blockade Borsippa. But the inhabitants "ridiculed the siege," and Cyrus could make no progress--such is the account in Herodotus and Xenophon. Owing to the amount of provisions at the command of the city, an investment could not promise any result, and there was little prospect of storming the city. The broad and deep trenches in front of the walls made it impossible to undermine them; even if these could be filled up under the missiles of the enemy in a few places for the battering-rams to be brought forward, the strength of the walls was so great that they could not be broken. Still less possible was it to mount them. They were so high that the arrows of the besiegers could not reach them with force, and even if the attack was carried successfully over the trenches, no towers or ladders would be at once strong and high enough to bring the storming party to the turrets. According to Herodotus, a long time had elapsed before Cyrus formed his plan. He bethought himself of the basin which Nebuchadnezzar had excavated at Sepharvaim, for the regulation of the inundations of the Euphrates, for feeding and damming up the canals; this work constructed for the benefit and protection of the land he used for the destruction of the capital. The Euphrates was to be led off into this basin till its bed could be forded at Babylon. Then the storming of the city was to be attempted from the river, the walls on the banks being less high and strong. For this object it was necessary to obtain possession of the fortress of Sepharvaim, which guarded the sluices of the basin, to deepen or enlarge the basin itself, so that for a certain period it could receive the whole mass of water; it was also requisite that the canal which led into it should be widened and deepened; and lastly the course of the river beneath the basin, or rather beneath the great canals which led into the Tigris, must be barred by a dam, if the Euphrates was to flow into it. The army of Cyrus must have been so strong, that after leaving behind a sufficient number of men on both