Jack Darrow fairly burst into the big shed where the two scientists
were at work over the ruined motor. They looked up at his excitable
entrance, and Mr. Henderson called out:

"Why, Jack, what's the matter?"

"Quite a lot, I'm afraid," answered the lad, and there was that in his
voice which alarmed the professors.

"What do you mean?" inquired Mr. Roumann, laying aside some of the
damaged motor plates.

"Mark's gone!" gasped Jack.

"Gone! Where?" exclaimed Mr. Henderson.

"I don't know, but he went to the deserted house, where we thought the
mysterious man was hiding, and since then I can't find him."

Then the frightened lad proceeded to explain what he and Mark had
undertaken, and the outcome of it; how his chum had failed to meet him
at the rendezvous, and how Jack had searched through the old house
without result.

"There's but one thing to do," declared Professor Henderson, when he
had listened to the story. "We must go back there and make a more
thorough search."

"What--to-night?" exclaimed the German.