There are also other passages of the ancients which are commonly supposed, but without certain grounds, to allude to water-mills. Among these is the following verse of Lucretius[400]: Ut fluvios versare rotas atque haustra videmus. It appears also that the water-wheels to which Heliogabalus caused some of his friends and parasites to be bound[401], cannot be considered as mills. These, as well as the _haustra_ of Lucretius, were machines for raising water, like those mentioned in the before-quoted passage of Vitruvius[402]. It is however evident that there were water-mills at Rome at this period; and it affords matter of surprise that we do not find mention oftener made of them, and that they did not entirely banish the use of the laborious hand- and cattle-mills. That this was not the case, and that the latter were very numerous for some time after, may be concluded from various circumstances. When Caligula, about twenty-three years after the death of Augustus, took away all the horses and cattle from the mills, in order to transport effects of every kind which he had seized, there arose a scarcity of bread at Rome; from which Beroaldus justly infers that water-mills must have been then very rare[403]. Nay, more than three hundred years after Augustus, cattle-mills were so common at Rome, that their number amounted to three hundred[404]. Mention of them, and of the hand-mills always occurs, therefore, for a long time after in the laws. The Jurist Paulus, who lived about the year 240, particularizing the bequest of a baker, mentions _asina molendaria_ and _mola_, a mill-ass and a mill[405]. In the year 319 Constantine ordered that all the slaves condemned to the mills should be brought from Sardinia to Rome[406]. Such orders respecting mill-slaves occur also under Valentinian[407]. When by the introduction of Christianity, however, the morals of men became improved, slaves were less frequent; and Ausonius, who lived under Theodosius the Great, about the end of the third century, expressly says, that in his time the practice had ceased of condemning criminals to slavery, and of causing mills to be driven by men.