What they saw was that the grounds seemed to be surrounded, near the wall, by heavy trees, which made it difficult to see much of what was within.  But in one place there was a break, so that, looking across velvety green lawns, they could see a small part of an old and weatherbeaten grey house.  It appeared to be on a rise, and to stand several stories above the ground, so that it might well be an ideal place for the establishment of a heliograph station.

But Harry’s suspicions were beginning to take a new turn.

“I believe this is the biggest find we’ve made yet, Dick,” he said.  “I think we’ll find that if we discover what is really going on here, we’ll be at the end of our task —­ or very near it.  It’s just the place for a headquarters.”

“I believe it is, Harry.  And if they’ve been so particular to keep everything about it secret, it certainly seems that there must be something important to hide,” suggested Harry, thinking deeply.  “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’s 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 tonight, 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 this steps.