"That's right," added Innis. "I don't want to get a call-down.
I'm about up to my limit now.

"We'll do it all right," announced Dick. "I haven't speeded the
motor yet. I've been warming it up. I'll show you what she can
do!"

He opened wider the gasoline throttle of the engine, and advanced
the timer. Instantly the boat shot ahead, as the motor ran at twice
the number of revolutions.

"That's something like!" cried Paul admiringly.

"She sure has got speed," murmured Innis.

On they sped, talking of the girls, of their plans for the summer,
and the coming examinations.

"Hark! What's that?" suddenly asked Paul, holding up his hand for
silence.

They were made aware of a curious, humming, throbbing sound.

"Some speed boat," ventured Dick.

"None in sight," objected Paul, with a glance up and down the river,
which at this point ran in a straight stretch for two miles or more.
"You could see a boat if you could hear it as plainly as that."