The ice had melted there to such an extent that only a thin glaze covered the surface. Through this transparent covering Frank Sheldon caught sight of what seemed to be the outline of a door covered with gravel that only partially concealed it. He thought he saw something too that faintly resembled an iron ring.
"A trap door!" he muttered under his breath, jubilant in the thought that he had perhaps fallen upon a clue to the mystery that had so long perplexed him.
He took out his knife and began to dig down toward the ring, when a low whistle from the opening to the alley warned him to be on the alert. Instantly the light was extinguished and the next moment Bart was at his side.
"Better let up, Frank," he whispered. "There's a big commotion down the street and a crowd is beginning to gather. I think it's a fire."
As he spoke, a fire engine clanged down the street and an increasingly red light made itself apparent in the sky.
"Too bad," grumbled Frank, as he put his flashlight back into his pocket "I think I was just on the verge of finding out something that would put us on the track of those fellows who seemed to vanish into thin air."
"Hard luck," murmured Bart, sympathetically, "but well have to give it up for the present."
Frank hesitated, but the increasing glare that made the alley visible and the sound of footsteps of people hurrying to the fire showed him that his friend was right, and he reluctantly desisted.
"To-morrow's a new day," said Bart consolingly, as the comrades stole out of the alley and mingled with the groups of passers-by.
"If Tom and Billy can tear themselves away from the girls, we'll bring them with us the first chance we get and try to clear up the whole mystery," observed Frank.