“Well, we don’t know all the ins and outs,” admitted Tom.  “Taking a midnight airship ride is rather strange, but that may have been the only course open.  We’ll have to let the explanation go until later.  At any rate, Mrs. Damon, I feel sure that your husband did go off through the air—­either in my Eagle or in some other craft.”

“Well, I’m glad to hear you say so, Tom Swift, though it sounds a dreadful thing to say.  But if he did go off of his own accord, I know he did it for the best.  And he may not have told me, for fear I would worry.  I can understand that.  But why isn’t he back now?”

Tom had been rather dreading that question.  It was one he had asked himself, and he had found no good answer for it.  If there had been such need of haste, that an airship had to be used. why had not Mr. Damon come back ere this?  Unless, as Tom feared to admit, even to himself, there had been some accident.

Half a dozen theories flashed through his mind, but he could not select a good, working one,—­particularly as there were no clues.  Disappearing in an airship was the one best means of not leaving a trace behind.  An auto, a motor boat, a train, a horse and carriage—­all these could be more or less easily traced.  But an airship—­

If Mr. Damon wanted to cover up his tracks, or if he had been taken away, and his captors wanted to baffle pursuit, the best means had been adopted.

“Now don’t you worry,” advised Tom to Mrs. Damon.  “I know it looks funny, but I think it will come out all right.  Ned and I will do all we can.  Mr. Damon must have known what he was about.  But, to be on the safe side, we’ll send out a general alarm through the police.”

“Oh, I don’t know what I’d done if you hadn’t come to help me!” exclaimed Mrs. Damon.

“Just you leave it to me!” said the young inventor, cheerfully.  “I’ll find Mr. Damon!”

But, though he spoke thus confidently, Tom Swift had not the slightest notion, just then, of how to set about his difficult task.  He had had hard problems to solve before, so he was not going to give up this one.  First he wanted to think matters out, and arrange a plan of action.

He and Ned made a careful examination of the grounds of the Damon homestead.  There was little they could learn, though they did find where an airship had landed in a meadow, not far away, and where it had made a flying start off again.

Carefully Tom looked at the marks made by the wheels of the airship.

“They’re the same distance apart as those on the Eagle,” he said to his chum, “and the tires are the same.  But that isn’t saying anything, as lots of airships have the same equipment.  So we won’t jump to any conclusions that way.”

Tom and Ned interviewed several of the neighbors, but beyond learning that some of them had heard the throbbing of the midnight airship, that was as far as they got on that line.

There was nothing more they could do in Waterford, and, leaving Mrs. Damon, who had summoned a relative to stay with her, the two chums made a quick trip back through the air to Shopton.  As Eradicate came out to help put away the monoplane Tom noticed that the colored man was holding one hand as though it hurt him.