not ten minutes later.

The land was far behind them now, and still in the other three
directions they saw only the level surface of the great lake.

His exclamation was called out by a sudden change in the method of
advance adopted by those in the leading aeroplane. Instead of keeping
along in a direct line the biplane had uptilted and was now shooting
downward in what seemed a terribly perilous way; just as though the pair
of precious scoundrels had taken a notion to end the pursuit by seeking
a plunge into the water.

But both boys knew differently, and that this was only a volplane,
adopted by experienced and rash aviators as a means of reaching the
lower air currents more rapidly than by slow spirals; or else undertaken
when having engine trouble that threatens destruction.

Frank was ready to follow suit. It would not be the first time by long
odds that the Bird boys had accomplished this speedy method of
descending from high altitudes. There was always an exhilaration about
the clever trick that appealed irresistibly to their natures; though
Frank would never have attempted it unless reasonably sure that the
conditions were favorable for success.

"Hold fast, Andy!" he remarked, quietly.

The hum of the motor suddenly ceased, and with its cessation the
hydroplane was turned head-on toward the surface of the lake, four
thousand feet below.

Down they went, plunging toward what seemed to be instant destruction;
but a steady hand was at the wheel, and the pilot knew just what was
necessary to do at the proper instant in order to bring this rapid
descent to a stop, and right the airship on a level keel.

But there was no time just then to note what the rival aeroplane might
be doing. Whether the experienced airman in charge managed to stop that
downward plunge before reaching the surface of the lake; or failing went
to his death, was a matter that did not concern them now, since they had
their own affairs to look after.

To tell the truth, Andy's heart seemed to be in his throat as they made
the drop. As yet they had never tried out the new hydroplane in a trick
of this sort; and hence really did not know just how it might act;
though Frank must have been pretty confident, else he would never have
attempted it.

Given the choice the Bird boys would easily have decided to descend from
their lofty height by means of the much safer if slower "spirals," each
circle seeing the aeroplane lower than before. But