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. The shore cables, which united the ships together, and sustained the actual bridge or platform, were made of most carefully selected materials, and must have been of enormous strength; the ships were placed in close proximity one to another; and by the substitution of a double for a single line--of two bridges, in fact, for one--the solidity of the work was very largely augmented. Yet, rare as was the skill shown, solid and compact as were the causeways thus thrown by human art over the sea, they were found inadequate to the end desired. The great work of Xerxes, far the most elaborate of its class, failed to withstand the fury of the elements for a single year; the bridge, constructed in one autumn, was utterly swept away in the next; and the army which had crossed into Europe by its aid had to embark as it best could, and return on board ship to Asia.