"You wait," was all Jack replied.

The projectile was slowed up to permit the two professors to make some
notes regarding a particularly large and deep crater, and a few minutes
later when Mark, who was in the engine room, attempted to speed up the
Cordite motor it would not respond.

"Humph! I wonder what's wrong?" he asked of Jack.

"Better call Mr. Roumann, and not try to fix it yourself," suggested
his chum, when, in response to various movements of the lever, the
machine seemed to go slower and slower.

The German came in answer to the summons.

"Ha!" he exclaimed, "that motor is broken again. We shall have to stop
once more for repairs. I shall need to take it all apart, I fear. Get
me the negative plate remover, will you, Mark?"

The lad went to the tool chest for it. He opened the lid and fumbled
about inside.

"It doesn't seem to be here," he announced.

"What! the negative plate remover not there?" cried the professor.
"Why, it must be. It is one of the new tools we got, and it has not
been used for anything; has it?".

"Oh, by Jinks!" cried Jack suddenly.