“No, it isn’t that.  But Mr. Foger lives there you know, and, though he may not be at home, there are probably some men who are interested in the thing he is working at.”

“You mean smuggling?”

“Well, I wouldn’t say that.  At the same time it may have leaked out that we are after the smugglers in an airship and it may be that Mr. Whitford doesn’t want the Fogers to know I’m on the ground until he has a chance to work up his clew.  So I’ll just go slowly, and remain in the background for a while.”

“Well, maybe it’s a good plan,” agreed Tom.

“Of course,” began Tom, “it would be—­”

He was interrupted by a shout from Koku, who had gone to the motor room, for the giant was as fascinated over machinery as a child.  As he yelled there came a grinding, pounding noise, and the big ship seemed to waver, to quiver in the void, and to settle toward the earth.

“Something’s happened!” cried Ned, as he sprang for the place where most of the mechanism was housed.

“Bless my toy balloon!” shouted Mr. Damon.  “We’re falling, Tom!”

It needed but a glance at the needle of the barograph, to show this.  Tom followed Ned at top speed, but ere either of them reached the engine room the pounding and grinding noises ceased, the airship began to mount upward again, and it seemed that the danger had passed.

“What can have happened?” gasped Tom.

“Come on, we’ll soon see,” said Ned, and they rushed on, followed by Mr. Damon, who was blessing things in a whisper.

The chums saw a moment later—­saw a strange sight—­for there was Koku, the giant, kneeling down on the floor of the motor room, with his big hands clasped over one of the braces of the bed-plate of the great air pump, which cooled the cylinders of the motor.  The pump had torn partly away from its fastenings.  Kneeling there, pressing down on the bed-plate with all his might, Koku was in grave danger, for the rod of the pump, plunging up and down, was within a fraction of an inch of his head, and, had he moved, the big taper pin, which held the plunger to the axle, would have struck his temple and probably would have killed him, for the pin, which held the plunger rigid, projected several inches from the smooth side of the rod.

“Koku, what is the matter?  Why are you there?” cried Tom, for he could see nothing wrong with the machinery now.  The airship was sailing on as before.

“Bolt break,” explained the giant briefly, for he had learned some engineering terms since he had been with Tom.  “Bolt that hold pump fast to floor crack off.  Pump him begin to jump up.  Make bad noise.  Koku hold him down, but pretty hard work.  Better put in new bolt, Mr. Tom.”

They could see the strain that was put upon the giant in his swelling veins and the muscles of his hands and arms, for they stood out knotted, and in bunches.  With all his great strength it was all Koku could do to hold the pump from tearing completely loose.