Such incidents as this, of personal communications between masses of enemies on the eve of a battle, were very common in ancient warfare, though impossible in modern times. In those days, when the missiles employed were thrown chiefly by the strength of the human arm alone, the combatants could safely draw near enough together for each side to hear the voices and to see the gesticulations of the other. Besiegers could advance sufficiently close to a castle or citadel to parley insultingly with the garrison upon the walls, and yet be safe from the showers of darts and arrows which were projected toward them in return. But all this is now changed. The reach of cannon, and even of musketry, is so long, that combatants, approaching a conflict, are kept at a very respectful distance apart, until the time arrives in which the actual engagement is to begin. They reconnoiter each other with spy-glasses from watch-towers on the walls, or from eminences in the field, but they can hold no communication except by a formal embassy, protected by a flag of truce, which, with its white and distant fluttering, as it slowly advances over the green fields, warns the gunners at the battery or on the bastion to point their artillery another way. The Babylonians, on the walls of their city, reproached and taunted their foes incessantly. "Take our advice," said they, "and go back where you came from. You will only lose your time in besieging Babylon. When mules have foals, you will take the city, and not till then." The expression "when mules have foals" was equivalent in those days to our proverbial phrase, "when the sky falls," being used to denote any thing impossible or absurd, inasmuch as mules, like other hybrid animals, do not produce young. It was thought in those times absolutely impossible that they should do so; but it is now well known that the case is not impossible, though very rare.