When Vitiges, king of the Goths, besieged Belisarius in Rome in 536, and caused the fourteen large aqueducts to be stopped, the city was subjected to great distress, not on account of the want of water in general, for it was secured against that inconvenience by the Tiber, but on account of the loss of that water which the baths required, and, above all, of that necessary to drive the mills, which were all situated on these canals. Horses and cattle, which might have been employed in grinding, were not to be found; but Belisarius, a man of great ingenuity, devised an expedient to remedy this distress. Below the bridge that reached to the wall of Janiculum, he extended ropes, well fastened, and stretched across the river from both banks. To these he affixed two boats of equal size, at the distance of two feet from each other, where the current flowed with the greatest rapidity, under the arch of the bridge; and, placing large millstones on one of the boats, suspended in the middle space a machine by which they were turned. He constructed at certain intervals on the river other machines of the same description, which, being put in motion by the force of the water that ran below them, drove as many mills as were necessary to grind provisions for the city. To destroy these, the besiegers threw into the stream logs of wood, and dead bodies, which floated down the river into the city; but the besieged, by making use of booms to stop them, were enabled to drag them out before they did any mischief. This is said to have been the first invention of floating mills.