"Some one evidently sneaked in here and loosened the breech-plug," went
on Mark, "and it was evidently done with the idea of delaying us. The
enemy could not have desired to utterly disable the projectile, or else
he would have tampered with the large motor, instead of the small one."

"Yes, the object seems to have been to delay us," admitted Professor
Henderson; "yet, I can't understand why. Whoever did it evidently knows
something about machinery."

"I hope they did not discover the secret of my Cardite motor," said
Professor Roumann quickly.

"They hardly had time," declared Mark. "We have been in or around the
projectile nearly every minute of the day, and whoever it was, must
have watched his chance, slipped in, stayed a few seconds, and then
slipped out again."

They went carefully over the entire projectile, but could find no
further damage done. Nor were there any traces of the person who had so
nearly caused a tragedy. Washington and Andy, after a careful search
outside the shed, had to admit that they had no clews.

"Well, the only thing to do is to go to work and build a new small
motor," announced Professor Roumann, after once more looking over the
_debris_ of the one that had exploded.

"Will it take long?" asked Jack.

"About two weeks. Fortunately, I can use some of the parts of this one,