came out of the old house. As he did so he gave a final call:

"Mark! Oh, you Mark! Are you anywhere about?"

What was that? Was it an answer, or merely the echo of his own voice?
Jack started, and then, as he heard another sound, he said:

"Only the wind squeaking a shutter. Mark isn't here."

If Jack had only known!

Through the quickly-gathering darkness Jack turned his steps toward
home. On the way along the country road he kept a sharp lookout for any
sign of his chum, and, also, he looked to see if he could catch a
glimpse of any person who might answer the description of the man they
suspected of tampering with the Cardite motor.

But the road was deserted, save for an occasional farmer urging his
horses along, that be might the more quickly get home to supper.

"It's mighty strange," mused Jack, as he kept on. "I don't think Mark
did just right, and yet, perhaps, when it's all explained, he may have
good reasons for what he did. Maybe I'm wrong to worry about him, and,
just as likely as not, he's safe home, wondering what kept me. But he
might have known that I'd come back to the barn where I said I'd meet
him. Of course that dog-fight delayed me a little, but not much."

It was quite dark when Jack reached the house where he and his chum
lived with the two professors. There was a cheerful light glowing from
many windows, and Jack also noticed an illumination in the shed where