Harry Girdwood dropped on one knee beside his companion and together they pressed the stone indicated in the singular cypher.
For a moment they felt no effect, but after a minute's effort they found that they had made an impression.
The discovery set them all aglow.
"Once more."
"Harder yet."
"Of course; only mind, Jack, no jerking."
"All right"
"We must work without making any noise; a jerk might bring down one of the stones with a clatter, which would alarm the guards.
"Caution is our watchword."
Soon they had the satisfaction of seeing the stone revolve and drop out into their arms.