hat 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."

"Not me, sir. I shall just shut one eye in the morning and let that
have a snooze for a couple of hours while I get on