“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.

“I’ll fix that in a jiffy,” said the driver, and began jacking up the wheel.  But, quickly as he stripped off the deflated tire, he was not so quick that Harry failed to see that the blow-out had been caused by a straight cut —­ not at all the sort of tear produced by a jagged stone or a piece of broken glass.  He said nothing of his discovery, however, and a moment later he looked up to face a young man in the uniform of an officer of the British territorial army.  This young man had keen, searching blue eyes, and very blond hair.  His upper lip was closely shaven, but it bore plain evidence that within a few days it had sported a moustache.

“Well,” said the officer, “what are you doing here?”

The driver straightened up as if in surprise.  “Blow-out, sir,” he said, touching his cap.  “I’m carrying these young gentlemen from Waterloo to Ealing, sir.  Had to come around on account of the roads.”

“You’ve have your way lost, my man.  Why not admit it?” said the officer, showing his white teeth in a smile.  He turned to Harry an Dick.  “Boy Scouts, I see,” he commented.  “You carry orders concerning the movement of troops from Ealing?  They are to entrain —­ where?”

“Near Croydon, sir, on the Brighton and South Coast Line,” said Harry, lifting his innocent eyes to his questioner.

“So!  They go to Dover, then, I suppose — no, perhaps to Folkestone —–­ oh, what matter?  Hurry up with your tire, my man!”

He watched them still as the car started.  Then he went back to the house.

“Whatever did you tell him that whopper about Croydon for?” whispered Dick.  “I wasn’t going to tell him anything -”