It likewise appears that the water-wheels to which Heliogabalus directed some of his friends and parasites to be tied, cannot be considered to be mills for the purpose of grinding corn; for these, as well as the _haustra_ of Lucretius were probably like those machines for raising water, which are spoken of by Vitruvius as _hydraulic_. It is, however, on the authority of Pompinius Sabinus, before-cited, that both wind and water mills were known to have been in Italy, and even the latter in Rome, in the days of Augustus. However, about twenty-three years after the death of Augustus, when Caligula seized every horse from the mills, to convey effects he had in contemplation to take from Rome, the public were much distressed for bread; whence we must infer that water-mills must have been very rare. Even three hundred years after Augustus, cattle mills were so common in that city, that their number amounted to three hundred; mention of them, and of the hand-mills, often occurs for a long time after. It is not their use we inquire after, it is enough for us to know that they existed. We now come to another period, when we are informed that _public mills_ were first introduced, which occurs in the year 398, mention being made of them in that year, which also clearly shows that they were then newly-established; which establishment was found necessary to be protected by laws made in their favour. The orders for that purpose were renewed more than once, and made more secure by Zeno, towards the end of the fifth century. It may be properly remarked, that in the whole code of Justinian, the least mention of wooden pales or posts is not made, which occurs in all the new laws,--and which, it appears, when there were several mills on the same stream, occasioned so many disputes then, as well as in after times. The mills at Rome were erected on those canals which conveyed water to the city; and because these were employed in several arts, and for many purposes, it was ordered that, by dividing the water, the mills should always be kept going; but as they were driven by so small a quantity of water, they probably executed very little work; and for this reason, but probably on account of the great number of slaves, and the cheap rate at which they were maintained, these noble machines were not so much used, nor were so soon brought to perfection, as under other circumstances they might have been. It appears, however, that after the abolition of slavery, they were much improved, and more employed, and to this a particular incident seems, in some degree, to have contributed.