e to word it so that it
would be hard for anyone except Colonel Throckmorton to understand it. And,
even after having purposely made the wording rather obscure, he put it into
code. And, after that, he thought of still another precaution that might
be wise.

"We won't need the credentials we've got in there to-night, Dick," he said.
"Nor our copies of the code, either. We'll bury them near where we leave
our motorcycles. Then when we get out we can easily get them back, and if
we should be caught they won't be found on us. Remember, if we are caught,
we're just boys out trespassing. Let them think we're poachers, if they
like."

But even Harry could think of no more precautions after that, and they had
a long and tiresome wait until they thought it was dark enough to venture
within the walls.

Getting over the wall was not difficult. They had thought they might find
broken glass on top, but there was nothing of the sort. Once inside,
however, they speedily discovered why that precaution was not taken--and
also that they had had a remarkably narrow escape. For scarcely had they
dropped to the ground and taken shelter when they saw a figure, carrying a
gun, approaching. It was a man making the rounds of the wall. While they
watched he met another man, also armed, and turned to retrace his steps.

"They've got two men, at least--maybe a lot more, doing that," whispered
Harry. "We've got to find out just how often he passes that spot. We want
to know if the intervals are regular, too, so that we can calculate just
when he'll be there."

Three times the man came and went, while they waited, timing him. And Harry
found that he passed the spot at which they had entered every fifteen
minutes. That was not exact for there was a variation of a minute or so,
but it seemed pretty certain that he would pass between thirteen and
seventeen minutes after the hour, and so on.

"So we'll know when it's safe to make a dash to get out," said Harry. "The
first thing a general does, you know, is to secure his retreat. He doesn't
expect to be beaten, but he wants to know that he can live to fight another
day if he is."

"We've got to retreat, haven't we?" said Dick. "It wouldn't do us any good
to stay here."

"That's so. But we've got to advance first. Now to get near that house,
and see what we can find. Look out for those traps and things our friend
warned us of. It looks like just the place for them. And keep to cover!"

They wo