The immediate purpose which this machine was to serve is clearly explained by all the ancient authors who mention it:  its use was to stop the enemy’s ships as soon as the Roman vessels came up with them, and thus to give them an opportunity of boarding them; but the construction and mode of operation of these machines it is not easy to ascertain from the descriptions of ancient authors.  Polybius gives the following description of them:  “They erected on the prow of their vessels a round piece of timber, about one foot and a half in diameter, and about twelve feet long, on the top of which a block or pully was fastened.  Round this piece of timber a stage or platform was constructed, four feet broad, and about eighteen feet long, which was strongly fastened with iron.  The entrance was lengthways, and it could be moved about the piece of timber, first described, as on a spindle, and could be hoisted within six feet of the top.  Round this there was a parapet, knee high, which was defended with upright bars of iron, sharpened at the end.  Towards the top there was a ring, through which a rope was fastened, by means of which they could raise and lower the engine at pleasure.  With this machine they attacked the enemy’s vessels, sometimes on their bow, and sometimes on their broadside.  When they had grappled the enemy with these iron spikes, if the ships happened to swing broadside to broadside, then the Romans boarded them from all parts; but when they were obliged to grapple them on the bow, they entered two and two, by the help of this engine, the foremost defending the forepart, and those who followed the flanks, keeping the boss of their bucklers level with the top of the parapet.”

From this description of the corvus, it is evident that it had two distinct uses to serve:  in the first place, to lay hold of and entangle the enemy’s ships; and, secondly, after it had accomplished this object, it served as a means of entering the enemy’s vessels, and also as a protection while the boarding was taking place.  With respect to the question, whether the harpagones or manus ferraeae; were the same with the corvi, it appears to us that the former were of much older invention, as they certainly were much more simple in their construction; and that, probably, the engineer who invented the corvi, borrowed his idea of them from the harpagones, and in fact incorporated the two machines in one engine.  The harpagones were undoubtedly grappling irons, but of such light construction that they could be thrown by manual force; but they were of no other service; whereas the corvi were worked by machinery, and served, as we have shown, not only to grapple, but to assist and protect the boarders.  We have been thus particular in our account of the corvus, because it may fairly be regarded as having essentially contributed to the establishment of the Roman naval power over that of the Carthaginians.