The boat was now in a part of the river where the land on both sides lies very low, behind very high levees. These are the richest cotton lands in the world, and their owners have tried to reclaim all of them from the river floods instead of taking only part of them for cultivation. Along other parts of the stream there are levees only here and there, leaving the river a chance to spread out over great areas of unreclaimed land, thus relieving the levees of much of the pressure upon them. Here, however, the line of embankment is continuous on both sides of the stream. For long distances the river is held between the two lines of artificially made banks. The water was now within a few inches of the top of the levees, and twenty or thirty feet above the level of the lands in the rear. The strain upon the embankments was almost inconceivably great, while the destruction which any break in that long line of earthworks would involve was appalling even to think of. The boys could see gangs of men at work wherever any weakness showed itself in the embankments, while sentinels, armed with shotguns, were everywhere on guard to prevent mischief-makers from cutting the levees. For, incredible as it may seem, men have sometimes been base enough to do this in order to let the river out of its banks, and thus reduce the danger of a break farther up stream where their own interests lay. For, of course, when a crevasse occurs at any point it lets so much water run suddenly out of the banks that the river falls several inches for many miles above, and the strain on the levees is greatly reduced. As the boys were floating down the middle of the flood, watching the work on the levees with keen interest, the air began to grow thick. A few minutes later a great bank of dense fog settled down upon them, covering all things as with a blanket. The shores and the great trees that grew upon them were blotted out. Then as the fog grew thicker and thicker, even the river disappeared, except a little patch of it immediately around the boat. On every side was an impenetrable wall of mist, and ragged fragments of it floated across the deck so that when they stood half the boat’s length apart the boys looked like spectres to each other. “I say, Phil, hadn’t we better go ashore or anchor?” said Constant. “Where is the shore?” asked Phil, quietly. “Why, there’s a shore on each side of us.” “Certainly. But in what direction? Which way is across the river, which way up the river, which way down the river?” “Why, the current will tell that,” said Constant. “How are we going to find out which way the current runs?” asked Phil, with a quizzical smile. “Easy enough; by looking at the driftwood floating by,” said the boy, going to the side of the boat to peer at the surface of the river through the fog. Presently he called out in amazement:--