and attacked the ropes on his feet. They were comparatively easy to
loosen, and soon he stood up unbound.

"Now for the secret panel!" he exclaimed, for he was convinced that it
was by some such means that his captor had entered and left. As has
already been explained, Mark knew on which side of his prison the
opening was likely to be--it would be where the warning knocks had
sounded. He began a minute inspection of that wall.

But if Mark hoped to speedily discover the secret he was doomed to
disappointment. He went over every inch of the surface, seemingly, and
pressed on every depression or projection that met his eye, as he
passed the candle flame along the wall.

Success did not reward him, and, as hour after hour passed, and the
candle burned lower and lower, Mark began to despair.

"I must escape before the projectile leaves," he murmured. "It will
never do to let them take that man with them under the impression that
they have me. I must escape! I will!"

Once more he began the tiresome task of seeking the secret spring. The
candle was spluttering in the socket now. It would burn hardly another
minute. Desperately Mark sought.

At last, just as the candle gave a dying gasp and flared brightly up
prior to going out, the lad saw a small screw head he had not noticed
before. It was sunk deep in a board.

"I'll press that and see what happens!" he exclaimed.