One of the garrison of the city, either in consequence of a dream or led of his own accord to it, collected together a great number of the workmen engaged in building the fortifications, and ordered them to dig a long trench in a certain place, which he pointed out to them, a considerable distance within the city wall, declaring that they would there find sweet water flowing out of the ground. He dug this trench in a circular form, making the depth of it for the most part about fifteen feet. This work proved the saving of the city, not through any foresight of the workmen, yet, by means of this trench, what would have been a misfortune was turned into a great advantage to the Romans; for, as during this time very heavy rain fell, the river, of which I just spoke, raged outside the fortifications and rose to a great height, being unable to proceed, because when it was swollen to such a size neither the channel nor the entrances in the wall were large enough to contain it, as they had been before. It consequently piled up its waters against the wall, rising to a great height and depth, and in some places was stagnant, and in others rough and violent. It at once overthrew the outwork, swept away a great part of the city wall, forced its way through the city gates, and, running with a great body of water, occupied almost the whole city, invading both the market-place, the narrower streets, and the houses themselves, swept off from them a great mass of furniture, wooden vessels, and such like things, and then, falling into this trench, disappeared under ground. Not many days afterwards it reappeared near to the city of Theodosiopolis, at a place about forty miles distant from the city of Dara, where it was recognised by the things which it had swept away out of the houses at Dara, for there the whole mass of them reappeared. Since that period, in time of peace and quiet, the river flows through the midst of the city, fills the reservoirs with water, and passes out of the city through the outfall especially constructed for it by the builders of the city, which I described above, and, as it supplies that region with water, becomes a great boon to the inhabitants of the neighbourhood. When, however, a hostile army advances to besiege the city, they close the passages through the iron gratings with what are called sluice-doors, and at once force the river to change its course and to flow into the trench and the subterranean gulf beyond, so that the enemy suffer from want of water and are at once obliged to raise the siege. Indeed Mirrhanes, the Persian general, in the reign of Cabades, came thither to besiege the city, and was compelled by all these difficulties to retire baffled after a short time; and a long while afterwards, Chosroes himself advanced with the same intentions, and attacked the city with a numerous army; but being driven to great straits through want of water, and perceiving the height of the walls,