The “boils” referred to are peculiar to the Mississippi, I believe. They are whirlpools, caused by the conflict of cross currents, and, as Will Moreraud said during this day of close watching, they are “sometimes right side up and sometimes upside down.” That is to say, sometimes a current from beneath comes to the surface like water in a boiling kettle and seems to pile itself up in a sort of mound for a half minute or so, while sometimes there is a genuine whirlpool strong enough even to suck a skiff down, as old-time flatboatmen used to testify. These were anxious hours for the young captain and his crew, but worse was to come. For night fell at last with the fog still on, and between the fog and the darkness it was no longer possible to see even the water at the sides of the boat from the deck. The crew had eaten no dinner that day. They had forgotten all about their meals in the eagerness of their watching. Now that watching was no longer possible they remembered their appetites, and had an evening dinner instead of supper. They set their lights of course, though it was of little use from any point of view. They could not be seen at a distance of twenty yards, and moreover there was nobody to see them. “There’s not much danger of any steamboat running into us now,” said Phil, who had carefully thought the matter out. “Why not?” asked Ed. “Because this fog has lasted for nearly twelve hours now, and by this time every steamboat is tied up to some bank or tree. For no pilot would think of running in such a cloud after finding any shore to which he could make his boat fast.” “But how can a steamboat find the shore when we can’t?” asked Will. “Because she can keep running till she finds it; and if she runs slowly she can back when she finds it till she makes an easy landing. She has power, and power gives her control of herself. We have none, except what the sweeps give us. In fogs like this steamboats always hunt for the shores and tie up till the fog lifts. So after ten or twelve hours of it, there are no steamboats prowling around to run into us.” “Another advantage the steamboats have in hunting for the shore,” said Will, “is that they can blow their whistles and listen for echoes. They can tell in that way not only in which direction the shore is, but about how far away it is.” “How do steamships manage in fogs out at sea?” asked Constant. “Theoretically,” replied Ed, “they slow down and blow their whistles or their ‘sirens,’ as they call the big steam fog-horns that can be heard for many miles. But in fact the big ocean steamships drive ahead at full speed--twenty miles an hour or more--blowing their sirens--till they hear some other ship’s siren. Then they act according to fixed rules, each ship turning her helm to port--that is to say to the left--so that they sail well away from each other.” “But what if there are sailing vessels in the way?”