They had not been here many days before a messenger came to them from the King of Macedon, which country, it will be seen, lies immediately north of Thessaly, earnestly dissuading them from attempting to make a stand at the Vale of Tempe. Xerxes was coming on, he said, with an immense and overwhelming force, one against which it would be utterly impossible for them to make good their defense at such a point as that. It would be far better for them to fall back to Thermopylae, which, being a narrower and more rugged pass, could be more easily defended. Besides this, the messenger said that it was possible for Xerxes to enter Thessaly without going through the Vale of Tempe at all. The country between Thessaly and Macedon was mountainous, but it was not impassable, and Xerxes would very probably come by that way. The only security, therefore, for the Greeks, would be to fall back and intrench themselves at Thermopylae. Nor was there any time to be lost. Xerxes was crossing the Hellespont, and the whole country was full of excitement and terror. The Greeks determined to act on this advice. They broke up their encampment at the Olympic Straits, and, retreating to the southward, established themselves at Thermopylae, to await there the coming of the conqueror. The people of Thessaly then surrendered to Xerxes as soon as they received his summons. Xerxes, from his encampment at Therma, where we left him at the close of the last chapter, saw the peaks of Olympus and Ossa in the southern horizon. They were distant perhaps fifty miles from where he stood. He inquired about them, and was told that the River Peneus flowed between them to the sea, and that through the same defile there lay the main entrance to Thessaly. He had previously determined to march his army round the other way, as the King of Macedon had suggested, but he said that he should like to see this defile. So he ordered a swift Sidonian galley to be prepared, and, taking with him suitable guides, and a fleet of other vessels in attendance on his galley, he sailed to the mouth of the Peneus, and, entering that river, he ascended it until he came to the defile. Seen from any of the lower elevations which projected from the bases of the mountains at the head of this defile, Thessaly lay spread out before the eye as one vast valley--level, verdant, fertile, and bounded by distant groups and ranges of mountains, which formed a blue and beautiful horizon on every side. Through the midst of this scene of rural loveliness the Peneus, with its countless branches, gracefully meandered, gathering the water from every part of the valley, and then pouring it forth in a deep and calm current through the gap in the mountains at the observer's feet. Xerxes asked his guides if it would be possible to find any other place where the waters of the Peneus could be conducted to the sea. They replied that it would not be, for the valley was bounded on every side by ranges of mountainous land.