"Go ahead, of course," said Dick. "In such work as this we rely upon the sergeant and you."
"Then I'd like to take a look at those heel prints also."
Dick thought he detected a quiver of excitement or emotion in the voice of Shepard, always so calm and steady hitherto, and he wondered. Nevertheless he asked no questions as he led the way out of the ravine.
The sergeant showed the heel prints to Shepard, and beyond question they had been made by a woman. By careful scrutiny they found a half dozen more leading in a diagonal direction up the side of the mountain, but beyond that the ground was so hard and rocky that they could discover no further traces.
"You agree with me that the tracks have just been made?" said the sergeant to Shepard.
"I do," replied the spy, his voice showing growing excitement, "and I think I know who made them. I didn't believe it at first. It seemed incredible. I want to try a little experiment. Will all of you remain perfectly still?"
"Of course," said Dick.
He took a small whistle from his pocket and blew upon it. The sound was not shrill like that of Slade's whistle, but was very low, soft and musical. He blew only a few notes. Then he took the whistle from his lips and waited. Dick saw that his excitement was growing. It showed clearly in the spy's eyes, and he felt his own excitement increasing, too. He divined that something extraordinary was going to happen.
Out of the cedars to their right and a little higher up the slope came the notes of a whistle, exactly similar, low, soft and musical.
"Ah, I knew it!" breathed Shepard. He waited perhaps half a minute and then blew again, notes similar and just the same in number. In a few moments came the reply, a precise duplicate.