The cab had a good engine, and it made good time. But after a little while Harry noticed with some curiosity that the route they were taking was not the most direct one. He rapped on the window glass and spoke to the driver about it.
"Got to go round, sir," the man explained. "Roads are all torn up the straight way, sir. Won't take much longer, sir."
Harry accepted the explanation. Indeed, it seemed reasonable enough. But some sixth sense warned him to keep his eyes open. And at last he decided that there could be no excuse for the way the cab was proceeding. It seemed to him that they were going miles out of the way, and decidedly in the wrong direction. He did not know London as well as a boy who had lived there all his life would have done. But his scout training had given him a remarkable ability to keep his bearings. And it needed no special knowledge to realize that the sun was on the wrong side of the cab for a course that was even moderately straight for Ealing.
They had swung well around, as a matter of fact, into a northwestern suburban section, and once he had seen a maze of railway tracks that meant, he was almost sure that they were passing near Willisden Junction. Only a few houses appeared in the section through which the cab was now racing and pavements were not frequent. He spoke to Dick: in a whisper.
"There's something funny here," he said. "But, no matter what happens pretend you think it's all right. Let anyone who speaks to us think we're foolish. It will be easier for us to get away then. And keep your eyes wide open, if we stop anywhere, so that you will be sure to know the place again!"
"Right!" said Dick.
Just then the cab, caught in a rutty road where the going was very heavy, and there was a slight upgrade in addition, to make it worse, slowed up considerably. And Dick, looking out the window on his side, gave a stifled exclamation.
"Look there, Harry!" he said. "Do you see the sun flashing on something on the roof of that house over there? What do you suppose that is?"
"Whew!" Harry whistled, "You ought to know that, Dick! A heliograph—field telegraph. Morse code—or some code—made by flashes. The sun catches a mirror or some sort of reflector, and it's just like a telegraph instrument, with dots and dashes, except that you work by sight instead of by sound. That is queer. Try to mark just where the house is, and so will I."
The cab turned, while they were still looking, and removed the house where the signalling was being done from their line of vision. But in a few moments there was a loud report that startled the scouts until they realized that a front tire had blown out. The driver stopped at once, and descended, seemingly much perturbed. And Harry and Dick, piling out to inspect the damage, started when they saw that they had stopped just outside the mysterious house.