When a dog, young and yet unskilled, follows his master across the meadows, it often happens that he meets with difficulties which sorely try the capacity of the inexperienced brain. The two come to a broad deep brook. The man glances at the opposite bank, and compares in his mind the distance to the other side with other distances he has previously leaped. The result is not quite satisfactory; somehow a latent doubt develops itself into a question of his ability to spring over. He cranes his neck, looks at the jump sideways to get an angular measurement, retires a few paces to run, shakes his head, deliberates, instinctively glances round as if for assistance or advice, and presently again advances to the edge. No; it will not do. He recalls to mind the division of space into yards, feet, and inches, and endeavours to apply it without a rule to the smooth surface of the water. He can judge a yard on the grass, because there is something to fix the eye on—the tall bennet or the buttercup yonder; but the water affords no data.

On second thoughts, yes—even the smooth flowing current has its marks. Here, not far from the steep bank, is a flag, bowed or broken, whose pennant-like tongue of green floats just beneath the surface, slowly vibrating to and fro, as you wave your hand in token of farewell. This is mark one—say three feet from the shore.