“Don’t you think you can catch them, Tom?” asked Mr. Damon.

“Well, I’m pretty sure I can, and yet they seem to have a faster biplane than I gave them credit for.  I guess I’ll have to increase our speed a little,” and he shifted a lever which made the Falcon shoot along at nearly doubled speed.

Still the other airship kept ahead, not far, but sufficiently so to prevent the grappling anchor from being tossed at her rail.

“I wonder if they are the smugglers?” questioned Mr. Damon.  “It might be possible, Tom, that we’re chasing the wrong craft.”

“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.