They crept along the hedge in the direction of the ridge. A third report rent the air; then, after a minute's silence, they were surprised to hear a renewed rumbling, which passed across their front nearer than they had heard it before, and receded towards the south.
"'Pon my word, it seems to have some connection with the gun after all," murmured Kenneth.
They went on, as fast as they could with caution. Crawling up the ridge, they peered over. Nothing was to be seen in either direction. They crawled down the other slope, and came to what appeared to be a sunken grass road. It was shadowed by the ridge. Looking to right and left, and discovering nothing, they got up and began to walk across the road. Suddenly Harry stumbled, and uttered a low exclamation.
"A whack on the toe," he murmured.
"By George!" whispered Kenneth behind him. He had stooped to look at the obstruction.
Harry turned. The obstacle was a rail. There was no glint from it; apparently it was rusty. But it was sticky to the touch. Kenneth held his fingers to his nose. They smelt of tar.
Beside the rail there was a layer of loose grass, twigs, rubbish of all sorts, and beyond this, five feet away, a parallel rail.
"We have come on a single-track railway," said Kenneth. "It's not marked on the map; must have been recently laid. Let us go on a little, and examine it."
In a few minutes their discovery was confirmed. The seeming grass road was a roughly laid track. But the rails had been painted over with tar, and the sleepers and permanent way were hidden under low heaps of litter.
"They're clever beasts," said Kenneth. "D'you see the trick? No airman would ever guess this to be a railway. The rails are quite dark."