Ian had seen the ladies pass, but made no effort to overtake them, although he was bound in the same direction: he preferred sauntering along with a book of ballads. Suddenly his attention was roused by a peculiar whistle, which he knew for that of Hector of the Stags: it was one of the few sounds he could make. Three times it was hurriedly repeated, and ere the third was over, Ian had discovered Hector high on a hill on the opposite side of the burn, waving his arms, and making eager signs to him. He stopped and set himself to understand. Hector was pointing with energy, but it was impossible to determine the exact direction: all that Ian could gather was, that his presence was wanted somewhere farther on. He resumed his walk therefore at a rapid pace, whereupon Hector pointed higher. There on the eastern horizon, towards the north, almost down upon the hills, Ian saw a congeries of clouds in strangest commotion, such as he had never before seen in any home latitude--a mass of darkly variegated vapours manifesting a peculiar and appalling unrest. It seemed tormented by a gyrating storm, twisting and contorting it with unceasing change. Now the gray came writhing out, now the black came bulging through, now a dirty brown smeared the ashy white, and now the blue shone calmly out from eternal distances. At the season he could hardly think it a thunderstorm, and stood absorbed in the unusual phenomenon. But again, louder and more hurried, came the whistling, and again he saw Hector gesticulating, more wildly than before. Then he knew that someone must be in want of help or succour, and set off running as hard as he could: he saw Hector keeping him in sight, and watching to give him further direction: perhaps the ladies had got into some difficulty! When he arrived at the opening of the valley just mentioned, Hector's gesticulations made it quite plain it was up there he must go; and as soon as he entered it, he saw that the cloudy turmoil was among the hills at its head. With that he began to suspect the danger the hunter feared, and almost the same instant heard the merry voices of the children. Running yet faster, he came in sight of them on the other side of the stream,--not a moment too soon. The valley was full of a dull roaring sound. He called to them as he ran, and the children saw and came running down toward him, followed by Mercy. She was not looking much concerned, for she thought it only the grumbling of distant thunder. But Ian saw, far up the valley, what looked like a low brown wall across it, and knew what it was. "Mercy!" he cried, "run up the side of the hill directly; you will be drowned--swept away if you do not." She looked incredulous, and glanced up the hill-side, but carne on as if to cross the burn and join him. "Do as I tell you," he cried, in a tone which few would have ventured to disregard, and turning darted across the channel toward her.