We had sailed along thus without incident for upward of half an hour, when my carelessness, or the darkness, nearly brought us to grief.  Suddenly, without warning, there was a violent jar and the raft rebounded with a force that all but threw us into the water.  Coming to a bend in the stream, the current had dashed us against the other bank.

But, owing to the flexibility of its sides, the raft escaped damage.  I had my oar against the wall instantly, shoving off, and we swung round and caught the current again round the curve.

But that bend was to the left, as the other had been, which meant that we were now going in exactly the opposite direction of that in which we had started!  Which, in turn, meant the death of hope; we were merely winding in and out in a circle and getting nowhere.  Harry and Desiree had apparently not noticed the fact, and I said nothing of it.  Time enough when they should find out for themselves; and besides, there was still a chance, though a slim one.

Soon the bed of the stream became nearly level, for we barely moved.  The roof of the tunnel was very low—­but a scant foot above our heads as we sat or crouched on the raft.  It was necessary to keep a sharp lookout ahead; a rock projecting from above would have swept us into the water.

The air, too, was close and foul; our breath became labored and difficult; and Desiree, half stifled and drowsy, passed into a fitful and broken sleep, stirring restlessly and panting for air.  Harry had taken the bow and I lay across the stern.  Suddenly his voice came, announcing that we had left the tunnel.

I sat up quickly and looked round.  The walls were no longer to be seen; we had evidently entered a cavern similar to the one in which we had embarked.

“Shall we lay off?” I asked, stepping across to Harry’s side.

He assented, and I took the oar and worked the raft over to the left.  There was but little current and she went well in.  In a few minutes we were in shallow water, and Harry and I jumped off and shoved her to the bank.

Desiree sat up, rubbing her eyes.

“Where are we?” she asked.

Harry explained while we beached the raft.  Then we broke out our provisions and partook of them.

“But why do we stop?” asked Desiree.

The words “Because we are not getting anywhere” rose to my lips, but I kept them back.

“For a rest and some air,” I answered.

Desiree exclaimed:  “But I want to go on!”

So as soon as we had eaten our fill we loaded the stuff again and prepared to shove off.  By that time I think Harry, too, had realized the hopelessness of our expedition, for he had lost all his enthusiasm; but he said nothing, nor did I. We secured Desiree on her pile of skins and again pushed out into the current.

The cavern was not large, for we had been under way but a few minutes when its wall loomed up ahead and the stream again entered a tunnel, so low and narrow that I hesitated about entering at all.  I consulted Harry.