According to Hal’s calculations, the spot where the aeroplane was hidden was far enough away so that the machine would not be disabled by the force of the explosion; and it was for this point that the lads made at full speed.

They reached there safely; and still there had been no explosion.

“How much time did you allow, Hal?” asked Chester.

“Ten minutes, as nearly as I could judge,” was the reply.

“Then we still have a few minutes, I guess.  Had we better wait here until after the blast, or shall we run out the machine and get up in the air.”

“We’d better stay here,” returned Hal, positively, “I don’t know how much ammunition there is in that barn.  It’s going to kick up a terrible fuss.  My advice is that we lay flat on the ground, hold our ears and bury our faces.  Immediately after the blast we’ll run the machine out and get up as swiftly as possible.”

“I can imagine the effect of the explosion,” said Chester.

“Well, I can’t,” returned Hal; “nor can you.  How many men it may kill, how many it may maim and what damage it will do cannot be estimated.  But one thing sure, immediately afterwards every sleepy German soldier within fifty miles will be on the alert.  The Germans will know it was not an accident.  They will attribute the explosion to a bomb dropped from the air.  We may have trouble reaching our lines.”

“I wish you hadn’t done it, Hal,” mumbled Stubbs, whom the lads had found hiding beside the aeroplane.  “It will dig a hole a mile deep in the ground.  Rocks, guns and everything will come down like hail.  We may be killed.”

“Quiet, Stubbs!” ordered Hal.  “Flat on the ground with you now.  Hold your ears and bury your faces until I tell you to get up.”

He suited the action to the word.  Chester and Stubbs followed his example.

For long moments, it seemed to them, they waited for the sound of the blast that would shake the country.  Each was anxious, for there was no telling what the result of the explosion might be.  Stubbs squirmed uneasily as he burrowed in the ground, while Chester and Hal were by no means easy in their minds.

So long did they wait that it seemed to Chester something must have gone wrong.  Perhaps the fuse had gone out.  Perhaps another German guard had discovered it in time and pinched out the fire.  There were many possibilities, and the lad considered them all as he lay prostrate on the ground.

He was about to raise his head and ask Hal a question, when, suddenly, the blast came.

There was, at first, a long grumbling roar, which, it seemed, would never end.  Gradually the roar increased until it reached such proportions as to be beyond all description; it was a roar the like of which neither of the three figures who lay there had ever heard before—­probably never would hear again.

Louder and louder it grew and then ended in a final blast that was louder than many thousand times the loudest peal of thunder—­louder than the simultaneous firing of thousands of guns.