“Much obliged,” said the young inventor to the purser, and then the two lads went back on deck.

A little later supper was served in the big dining saloon, and the boys and Mr. Damon were glad of it, for they were hungry.  Eradicate ate with a party of colored persons whose acquaintance he had quickly made.  It was a gay gathering in which Tom and Ned found themselves, for though they had traveled much, generally it had been in one of Tom’s airships, or big autos, and this dining on a big ship was rather a novelty to them.

The food was good, the service prompt, and Tom found himself possessed of a very good appetite.  He glanced across the table and noted that opposite him and Ned, and a little way down the board, were two vacant chairs.

“Can’t be that anyone is seasick already.” he remarked to his chum.

“I shouldn’t think so, for we haven’t any more motion than a ferryboat.  But some persons are very soon made ill on the water.”

“If they’re beginning thus early, what will happen when we get out where it’s real rough?” Tom wanted to know.

“They’ll sure be in for it,” agreed Ned, and a glance around the dining saloon showed that those two vacant chairs were the only ones.

Somehow Tom felt a vague sense of uneasiness—­as if something was about to happen.  In a way he connected it with the suspicion that the Fogers were aboard, and with his subsequent discovery that their names were not on the passenger list.  Then, with another thought in mind, he looked about to see if be could pick out the man and youth who, on coming up the gang plank, had been taken by both Tom and Ned to be their enemies.  No one looking like either was to be seen, and Tom’s mind at once went back to the vacant seats at the table.

“By Jove, Ned!” he exclaimed.  “I believe I have it!”

“Have what—­a fit of seasickness?”

“No, but these empty seats—­the persons we saw you know—­they belong there and they’re afraid to come out and be seen.”

“Why should they be—­if they’re not the Fogers.  I guess you’ve got another think coming.”

“Well, I’m sure there’s something mysterious about those two—­the way they hid their faces as they came on board—­not appearing at supper—­I’m going to keep my eyes open.”

“All right, go as far as you like and I’m with you.  Just now you may pass me the powdered sugar.  I want some on this pie.”

Tom laughed at Ned’s matter-of-fact indifference, but when the young inventor turned in to his berth that night he could not stop thinking of the empty seats—­the two mysterious passengers—­and the two Fogers.  They got all jumbled in his head and made his sleep restless.

Morning saw the Maderia well out to sea, and, as there was quite a swell on, the vessel rolled and pitched to an uncomfortable degree.  This did not bother Tom and Ned, who were used to sudden changes of equilibrium from their voyages in the air.  Nor did Mr. Damon suffer.  In fact he was feeling fine and went about on deck like an old salt, blessing so many new things that he had many of the passengers amused.