Although the Romans were slow in becoming a great naval power, it is clear that very soon after the battle of Actium they became as powerful at sea as on land; all the leading seafaring states having by this time become subject to the engrossing power of the empire, and there being no longer any naval power to contest with them the supremacy of the Mediterranean Sea: hence, for many years, there was no employment for their fleets, except to protect their commerce and to keep open the communication with the distant colonies of the empire. But long after Rome had swept the sea of her greater opponents, marauders of different races continued to prey upon commerce, so that her merchant traders, engaged on distant voyages, invariably took on board a number of Roman archers to guard their ships from pirates. These men were engaged either to protect the ship when an emergency arose,[320] or to man the oars, and, like the marines of our own time, acted as soldiers, or performed those portions of the sailor’s duties to which they had been trained. [Sidenote: The corn trade of the city.] Although, unfortunately, no clearly defined records of the maritime commerce of Rome at any period of her history have been preserved, its extent may be conjectured by the number and wealth of her citizens. One million and a quarter of people,[321] including nearly all the wealthy inhabitants of the empire—for these flocked to the capital in great numbers—would at any time require a large fleet of merchant ships to supply their varied wants. Even during the time of the Roman commonwealth, the vessels engaged in the corn trade alone of the city were so numerous, that medals were struck in commemoration of the traffic, bearing upon them the representation of the prow of a ship, and the inscription, “By decree of the Senate, for the purchase of corn.” That trade, as well as all others, increasing with the spread of the metropolis, must, in the reign of the Cæsars, have required an amount of shipping to supply its wants far in excess of that of any other city of either ancient or modern times. If one steamship now performs the work of at least three sailing vessels, each of equal capacity to the steamer, then, considering the dilatory movements of ancient vessels, it will not be an exaggerated estimate to allow six thousand tons of shipping as necessary to conduct the same amount of transport as one steamship of a thousand tons would now perform. Allowing, therefore, that Rome was not one half the size of London, and presuming that it required one half the amount of supplies from abroad, the shipping employed in its over-sea trade would be more than twice as great as that now necessary to supply the wants of the inhabitants of the metropolis of Great Britain. [Sidenote: A.D. 41.] [Sidenote: Port of Ostia.]