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.
With a suddenness that was startling, he found himself in total darkness. The candle had burned out, but he had his finger on the screw. He pressed it with all his force.
There was a rumbling sound in the darkness, a movement as if some heavy body had slid out of the way, and Mark felt a breath of air on his cheeks. Then he saw a dim light.
"Oh, I'm out! I'm out!" he cried joyously, breathing a prayer of thankfulness at his deliverance. "I'm free! I pushed on the right spring, and the panel slid back!"
He fairly leaped forward. The morning light was streaming in through the broken windows. He saw himself in the old hall of the mansion, at the head of the stairs, in a sort of anteroom, the mantle of which apartment had swung aside to give him egress from the secret chamber through a hole in the wall. He was free!