The distinct and well-known "popping" of a motor was heard.

"It's a trick, fellows! Somebody is meddling with our machines! Back to
the road!" shouted Jerry, turning and plunging through the under-brush
recklessly.

A wild scramble followed. The four chums were so excited, and filled
with a determination to stop the unknown miscreants from making way
with their machines, that they gave little heed to their steps. The
consequence was that more than once a collision with a tree ensued, and
various bumps afterward gave mute evidence as to the reckless manner of
their chase.

"There's two of 'em!" shrieked Will from the rear, as he caught the
sound of a second series of erratic poppings.

Evidently those who were meddling with the motorcycles did not have a
thorough knowledge of how to work the same, for the sounds would
suddenly cease and then start up again.

"Oh! don't I wish they'd just take headers over some nice fat root!"
gasped the perspiring Will, still hugging his precious camera to his
heart as he followed in Frank's wake.

The latter had made for the road in as direct a line as possible.
Progress was bound to be slow through the dense undergrowth, and the
sooner they struck the open the quicker they could hope to gain on the
thieves.