"All right. I'm with you."
By the time they had reached the alley it was almost entirely dark. Choosing a moment when the street was empty, they slipped into the alley and made their way toward the further end.
They felt the walls on either side as they went along for indications of a door or opening of any kind. They did the same with the blank wall that closed the alley at the other end. Nothing rewarded their search. The wall at the farther end was far too high to scale. It seemed impossible for anything except with wings to vanish from the alley as completely as had their assailants on that memorable night when they had so nearly lost their lives.
"It beats me," said Bart at length. "We saw them go in and we followed them up and they weren't there. Sounds like black magic, doesn't it?"
"It surely does," agreed Frank, in great perplexity. "They didn't go through the back, they couldn't go through the sides, they couldn't go into the air, but they did go somewhere."
"Down into the ground," suggested Bart jokingly. "That seems the only place left."
Frank started.
"There's many a true word spoken in jest," he said. "Perhaps you've hit it, Bart. That's one place we haven't examined."
"Small chance to examine that just now," said Bart. "You can see it's all covered with a glare of ice. There isn't a bit of ground showing."
They walked over the ice-covered surface with scarcely a hope under present conditions of making any discoveries, even if there were any to make. They had to depend entirely upon the sense of touch, for it was by this time pitch dark and Frank did not care to flash his light for fear they might be observed by passers-by.