It will take a skilled mind like yours to see where the fault lies."

Jack explained volubly, while David Pollard looked over the model that
the trio of young geniuses had put together. Then Benson drew from an
inner pocket, and spread out, some carefully made mechanical drawings
that made his idea plainer. Jack was not a trained draughtsman, but he
had a great natural talent in that direction.

"Why, you have a splendid idea here," cried the inventor, presently.

"It doesn't quite work, though," said Hal, ruefully.

"Lot's of inventions don't, unfortunately," winced David Pollard. "I
know something about that, for a big percentage of my inventions have
turned out to have more flaws than good points. But this is really
ingenious, boys. Who has had the big share in this get-up?"

"The other fellows," replied the young captain.

"Jack's idea, mostly," broke in Eph, "although Hal Hastings and I have
been allowed to butt in some."

"It's splendidly done, as far as you've gone," glowed the inventor, full
of unselfish admiration. "And you've made it plain just how you expect
to attach this device and make it work automatically. What are you going
to do with it, now?"

"We thought, perhaps, Mr. Pollard," explained Captain Jack, "that you
might think it worth while to take the device up at this point, and work
over it until you find out where the hitch is in the idea. If you