"They must have been ghosts then," gibed Tom. "Nothing else could
have vanished through a brick wall."

Frank drew his flashlight from his pocket and flashed it about.
There was no one to be seen.

"That wall is perfectly blank," he murmured in perplexity. "Thirty
feet high if it's an inch. There isn't an opening in it anywhere."

"Could they have got into the windows of the building on either
side?" suggested Bart.

Frank swept the flashlight on the walls of the factories.

"Not a chance," he affirmed. "All these windows are protected with
iron bars and nothing could get between them. Those fellows seem
to have just melted away."

At that instant a report rang out, and the flashlight was knocked
from his hand by a bullet.

"Down, fellows!" he shouted, setting the example, and the next
moment all four were lying flat on the ground.

They were just in time, for there was a crackling of guns, and
other bullets sped over their heads.

"After them, boys!" yelled Frank, leaping to his feet. "They're at
the mouth of the alley. I saw the flash from their guns."