"How far away do you think it ought to be, Harry?" asked Dick.
"It's very hard to tell, Dick. A first-class heliograph is visible for a very long way, if the conditions are right. That is, if the sun is out and the ground is level. In South Africa, for instance, or in Egypt, it would work for nearly a hundred miles, or maybe even more. But here I should think eight or ten miles would be the limit. And it's cloudy so often that it must be very uncertain."
"Why don't they use flags, then?"
"The way we do in the scouts? Well, I guess that's because the heliograph is so much more secret. You see, with the heliograph the flashes are centered. You've got to be almost on a direct line with them, or not more than fifty yards off the centre line, to see them at all, even a mile away. But anyone can see flags, and read messages, unless they're in code. And if these people are German spies, the code wouldn't help them. Having it discovered that they were sending messages at all would spoil their plans."
"I see. Of course, though—that's just what you said. It was really just by accident that we saw them flashing."
Then they came to the house where they expected to make their observation. It was occupied by an old gentleman, who came out to see what was wanted and stood behind the servant who opened the door. At the sight of their uniforms he drew himself up very straight, and saluted. But, formal as he was, there was a smile in his eyes.
"Well, boys," he said, "what can I do for you? On His Majesty's service, I suppose?"
"Yes, sir," said Dick. "We'd like to go up in your tower room, if you don't mind."
"Scouting, eh?" said the old gentleman, mystified. "Do you expect to locate the enemy's cavalry from my tower room? Well, well—up with you! You can do no harm."
Dick was inclined to resent the old gentleman's failure to take them seriously, but Harry silenced his protest. As they went up the stairs he whispered: "It's better for him to think that. We don't want anyone to know what we're doing, you know—not yet."