When Vitiges, King of the Goths, besieged Belisarius in Rome, in the year 536, and caused the fourteen large expensive aqueducts to be stopped, the city was reduced to great distress; not from want of water, in general, because it was secured against that inconvenience by the Tiber; but by 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 upon grinding, were not to be found; but Belisarius fell upon the ingenious contrivance of placing boats upon the Tiber, on which he erected mills that were driven by the current. This experiment was attended with complete success; and as many mills of this kind as were necessary were constructed. To destroy these, the besiegers threw into the stream logs of wood, dead bodies, &c., 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 could do any mischief. This seems to have been the origin of _floating-mills_, no record of them appearing previously. By these means the use of water-mills became very much extended; for floating-mills can be constructed almost upon any stream, without forming an artificial fall; they may be stationed at the most convenient places, and they rise and fall of themselves with the water.--They are, however, attended with these inconveniences: they require to be strongly secured; they often block up the stream too much, and move slowly; and they often stop when the water is too high, or when it is frozen. After this improvement, the use of water-mills was never laid aside or forgotten, but was soon made known all over Europe; and passages innumerable might be quoted, in every century, to prove their continued use. The Roman, Salic, and other laws, constantly provided for the security of these mills, and defined a punishment for such as destroyed the sluices, or stole the mill-irons. It is said, however, that there were water-mills in France and Germany a hundred years before these laws had existence. At Venice, and other places, there were erected mills which regulated themselves by the motion of the waters, and which were regulated by the flowing and ebbing of the tide, and which every six hours changed the motion of the wheels. Of this species of mills, a new invention, or, perhaps, rather an improved one, was made in London, called a tide-mill, an engraving of which may be seen in “The Advancement of Arts, Manufactures, and Commerce,” London, 1772. Zanetti is said to have shown, by some old charters, that such mills existed about the year 1044; but with still more certainty in 1078, 1079, and 1107.