"I think I'll write a letter to Colonel Throckmorton, Dick. I'll tell him about this place, and that we're trying to get in and find out what we can about it. Then, if anything happens to us, he'll know what we were doing, and he will have heard about this place, even if they catch us. I'll post it before we go in."
"That's a splendid idea, Harry. I don't see how you think of everything the way you do."
"I think it's because my father's always talking about how one ought to think of all the things that can go wrong. He says that the way he's got along in business is by never being surprised by having something unfortunate happen, and by always trying to be ready to make it as trifling as it can be."
So Harry wrote and posted his letter, taking care 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."