He had need to recognize at a glance the ripple on the water that told of a lurking sand-bar and distinguish it from the almost identical ripple that a brisk breeze would raise. Most perplexing of the perils that beset river navigation are the "snags," or sunken logs that often obstruct the channel. Some towering oak or pine, growing in lusty strength for its half-century or more by the brink of the upper reaches of one of the Mississippi system would, in time, be undermined by the flood and fall into the rushing tide. For weeks it would be rolled along the shallows; its leaves and twigs rotting off, its smaller branches breaking short, until at last, hundreds of miles, perhaps, below the scene of its fall, it would lodge fair in the channel. The gnarled and matted mass of boughs would ordinarily cling like an anchor to the sandy bottom, while the buoyant trunk, as though struggling to break away, would strain upward obliquely to within a few inches of the surface of the muddy water, which--too thick to drink and too thin to plough, as the old saying went--gave no hint of this concealed peril; but the boat running fairly upon it, would have her bows stove in and go quickly to the bottom. After the United States took control of the river and began spending its millions annually in improving it for navigation and protecting the surrounding country against its overflows, "snag-boats" were put on the river, equipped with special machinery for dragging these fallen forest giants from the channel, so that of late years accidents from this cause have been rare. But for many years the riverman's chief reliance was that curious instinct or second sight which enabled the trained pilot to pick his way along the most tortuous channel in the densest fog, or to find the landing of some obscure plantation on a night blacker than the blackest of the roustabouts, who moved lively to the incessant cursing of the mate.