This time there could be no doubt. He plainly heard a dry twig snap
near by; whether it were under the weight of man or beast, he did not
know.

"There is something out there. It couldn't have been the pie after
all. I'm going to find out what it is before I get back into this bed
again," he decided firmly, slipping quietly from under the covers and
peering out through the half closed flap of his tent.

As before, all was silence, the drowsy, indistinct voices of the night
passing almost without notice.

But Professor Zepplin instead of waiting where he was, reached for his
revolver and then strode boldly out into the open space in front of
the tents, determined to solve the mystery, and, if possible, without
waking the boys.

The reader no doubt already has recognized in the four boys sleeping
in the little weather-beaten tents the same lads who some time before
had started off for a vacation in the mountains where they hunted the
cougar and the bobcat, the thrilling adventures met with on that
journey having been related in a former volume entitled, "THE PONY
RIDER BOYS IN THE ROCKIES."

They will be remembered, too, as the lads who, in "THE PONY RIDER BOYS
IN TEXAS," crossed the plains on a cattle drive, during the course of
which Tad Butler bravely saved the life of the Chinese cook, by
plunging into a swollen torrent; and later, saved a large part of the
great herd, himself being nearly trampled to death in a wild stampede