"Him go fast," grinned the Mexican.

"So can a crab on dry land," jeered Ned.

Just then the guide utter a series of shrill "yi-yi's," whereupon the
lads were treated to an exhibition such as they never had seen before.

The sleepy burro projected his head straight out before him, while his
tail, raised to a level with his back, stuck straight out behind him.
The burro, seemingly imbued with sudden life, was off at a pace faster
than a man could run.

It was most astonishing. The boys gazed in amazement; then burst out
in a chorus of approving yells.

But it was the rider, even more than the burro, that excited their
mirth. His long legs were working like those of a jumping jack, and
though astride of the burro, Juan was walking at a lively pace. It
reminded one of the way men propelled the old-fashioned velocipedes
years before.

A cloud of dust rose behind the odd outfit as the party drew out on
the plains. Their ponies were started at a gallop, which was necessary
to enable them to keep up with the pace that Juan had set.

"Here! Here!" shouted the Professor.

Juan never looked back.

"We're leaving the pack train. Slow down!"