As the time approached, Mark, in spite of his assumed cheerfulness, could not master a slight feeling of discomfort. It was evidently going to be a cool, dark night. The very sound of it was startling to the lad--the announcement that he was to keep the full watch over their little camp of two waggons in a country where lions were common, and on one of the banks of the river which might very well be haunted by hippopotami and loathsome crocodiles. The captain had spoken of its being called Reptile River, and of course that was what it meant. The very thought of it was alarming. He had read enough to know that hippopotami came out to feed by night, crushing up the succulent weeds and softer canes, grinding all up in their huge portmanteau-like jaws, while it was a well known fact that the ponderous beasts would rush at and trample down anyone who came in their way. All that was bad enough, but nothing to compare for horror with the thought of a huge lizard or newt-shaped creature lying in wait ready to seize upon human being or ordinary animal, and drag its prey down into some hole beneath the bank, ready to be devoured at the monster's leisure. Mark tried very hard to chase away such thoughts, but they kept coming on, right up to the time when he finished his supper and met the doctor's eyes. "Ready, Mark?" he said. "Yes, sir," said the boy firmly. "That's right," said his father. "Keep a sharp look out, my boy. We are all trusting you to take care of us for the next few hours. Good-night." "Good-night, father." The boy shouldered the double rifle and followed the doctor. "I should keep moving, Mark, my boy," said the latter. "It will occupy your attention and make the time seem to pass more quickly. It will keep you warm too, for it's sure to be very chilly later on. Stop here a minute or two. I just want to go to the forward waggon and say a word or two to the men. I will join you again directly." Mark drew himself up stiffly as the doctor walked away, and then his heart seemed to give a bound, for there was a faint rustle just in front, and the boy brought his piece down to the present and made the locks click. "It's only me, sir," said a familiar voice. "You, Dan!" cried the boy, pressing one hand on the region of his heart, which was beating fast. "Yes, sir; just me. You need not shoot. I have been waiting till the doctor had gone. I thought as it was rather a new job for a youngster like you, I'd come and ask you whether you would like me to come and keep the watch with you. You see, it's all fresh to you, but it's the sort of thing I have been brought up to aboard ship." "It is very good of you," said Mark, warming up at the man's thoughtfulness, "but you had better go and lie down and go to sleep." "I don't want to go to sleep, sir." "But you will lose your night's rest."