"Possible, but not probable," put in Mr. Whitford. "After the clew
we got, and what the Indians told us, and then to have a biplane
come sailing over our heads at night, it's pretty sure to be the one
we want. But, Tom, can't you close up on 'em?"

"I'm going to try. The machinery is warmed up now, and I'll send it
to the limit."

Once more he adjusted the wheels and levers, and at his touch the
Falcon seemed to gain new strength. She fairly soared through the
air.

Eagerly those in the pilot house watched the craft they were
pursuing. She could be seen, in the glare of the big searchlight,
like some bird of gloom and evil omen, fluttering along ahead of
them.

"They certainly have a fine motor!" cried Tom. "I was sure I could
have caught up to them before this."

"How do you account for it?" asked Mr. Damon.

"Well, they're flying a good deal lighter than we are. They probably
have no load to speak of, while we carry a heavy one, to say nothing
of Koku."

"Diamonds aren't very heavy," put in Mr. Whitford grimly. "I think
they are smuggling diamonds to-night. How I wish we could catch
them, or trace them to where they have their headquarters."

"We'll do it!" declared Tom.

"Bless my stars! They've gone!" suddenly exclaimed Mr. Damon.
"They've disappeared, Tom, I can't see them."

It was indeed true. Those in the pilot house peering ahead through
the darkness, could not get a glimpse of the airship they were
pursuing. The beam of the searchlight showed nothing but a black
void.

All at once the beam shifted downward, and then it picked up the
white-winged craft.

"They went down!" cried Tom. "They tried to drop out of sight."

"Can't you get them?" asked Mr. Whitford.

"Oh, yes, we can play that game too. I'll do a little volplaning
myself," and the young inventor shut off the power and coasted
earthward, while Ned, who had picked up the forward craft, kept the
searchlight playing on her.

And now began a wonderful chase. The smugglers' craft, for such she
proved later to be, did her best to dodge the Falcon. Those managing
the mechanism of the fleeing airship must have been experts, to hold
out as they did against Tom Swift, but they had this advantage, that
their craft was much lighter, and more powerfully engined as regards
her weight. Then, too, there were not so many on board, and Tom,
having a combined balloon and aeroplane, had to carry much
machinery.

It was like the flight of two bi