human
How far away do you think it ought to be, Harry?
gpt
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.
human
Why don't they use flags, then?
gpt
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.
human
I see. Of course, though--that's just what you said. It was really just by accident that we saw them flashing.