had decided, they should be able to get the view they required and
so locate the second heliographing station.

"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