Probably, they varied very considerably in their size and burthen, including huge and heavy merchantmen on the one hand, and a much lighter and smaller craft on the other.
The Persians used their ships of war, not only for naval engagements, but also for the conveyance of troops and the construction of bridges.
Accustomed to pass the great streams which intersect Western Asia by bridges of boats, which were permanently established wherever an unfordable river crossed any of the regular routes connecting the provinces with the capital, the Persians, when they proceeded to carry their arms from Asia into Europe, conceived the idea of bridging the interval between the continents, which did not much exceed the width of one of the Mesopotamian streams, by constructions similar in principle and general character to those wherewith long use had made them familiar in their own country.

Ranging a number of vessels side by side, at no great distance one from another, parallel with the course of the stream, which ran down the straits, anchoring each vessel stem and stern to keep it in place, and then laying upon these supports a long wooden platform, they made a floating bridge of considerable strength, reaching from the Asiatic to the European coast, on which not only men, but horses, camels, chariots, and laden carts passed over safely from the one continent to the other.

Only, as the water which they had to cross was not a river, but an arm of the real salt sea, and might, therefore, in case of a storm, show a might and fury far beyond a river's power, they thought it necessary to employ, in lieu of boats, the strongest ships which they possessed, namely, triremes and pentecon-ters, as best capable of withstanding the force of an angry sea.

Bridges of this kind were intended sometimes for temporary, sometimes for permanent constructions.

In the latter case, great care and much engineering skill was lavished on their erection.