In the third and greatest of the expeditions conducted by Persia against Greece, the Phoenicians are found to have played a very important and prominent part. Even before the expedition commenced, a call was made upon them in connection with it for services of an unusual character. The loss of the fleet of Mardonius off Mount Athos induced Xerxes to determine on cutting a ship-canal through the isthmus which joins Athos to the mainland; and his passion for great and striking achievements caused him to project the construction of a double bridge of boats across the Hellespont. Phoenician technical skill was invoked for the furtherance of both objects. At Athos they worked in conjunction with the maritime states generally, but showed an amount of engineering knowledge far in advance of their fellow-labourers. The others attempted to give perpendicular sides to their portions of the excavation, but found the sides continually fall in, and so (as Herodotus observes) “had double labour.”[14296] The Phoenicians alone knew that the sides must be sloped at an angle, and, calculating the proper slope aright, performed their share of the task without mishap. At the Hellespont the Phoenicians had for co-partners the Egyptians only, and the two nations appear to have displayed an equal ability.[14297] Cables were passed from shore to shore, made taut by capstans and supported by an almost continuous line of boats; planks were then laid upon the cables, and covered with brushwood, while a thick layer of earth was placed upon the top. A solid causeway was thus formed, which was guarded on either side by bulwarks of such a height that the horses which crossed the bridge could not see over them; and thus the cavalry and the sumpter beasts passed from one continent to the other without a suspicion that they had ever had anything but _terra firma_ under them. The structure served its purpose, but was not found strong enough to defy even for a year the forces of the winds and waves. Before the return of Xerxes, towards the close of B.C. 480, the autumnal gales had broken it up; and the army which accompanied him had to re-cross the strait in a number of separate ships.[14298]