stons. There were sixteen of these cylinders, and
the pistons all connected with a small engine invented by Castellan,
which he called an accelerator. By means of this device he could
regulate the speed of the propellers which drove the vessel under water
and in the air from sixty up to two thousand revolutions a minute.

The _Flying Fish_ was driven by nine propellers, three of these,
four-bladed and six feet diameter, revolved a little forward amidships
on either side under what might be called the fins. These fins collapsed
close against the sides of the vessel when under water and expanded to a
spread of twenty feet when she took the air. They worked on a pivot and
could be inclined either way from the horizontal to an angle of thirty
degrees. Midway between the end of these and the stern was a smaller
pair with one driving screw. The eighth screw was an ordinary propeller
at the stern, but the outside portion of the shaft worked on a ball and
socket joint so that it could be used for both steering and driving
purposes. It was in fact the tail of the _Flying Fish_. Steering in the
air was effected by means of a vertical fin placed right aft.

She was submerged as the _Ithuriel_ was, by pumping water into the lower
part of her hull. When these chambers were empty she floated like a
cork. The difference between swimming and flying was merely the
difference between the revolutions of the screws and the inclination of
the fins. A thousand raised her from the water: twelve hundred gave her
twenty-five or thirty miles an hour through the air: fifteen hundred
gave her fifty, and two thousand gave her eighty to a hundred, according
to the state of the atmosphere.

Her armament consisted of four torpedo tubes which swung at any angle
from the horizontal to the vertical and so were capable of use both
under water and in the air. They discharged a small,
insignificant-looking torpedo containing twenty pounds of an explosive,
discovered almost accidentally by Castellan and known only to himself,
the German Emperor, the Chancellor, and the Commander-in-Chief. It was
this which he had used in tiny quantities in the experiment at Potsdam.
Its action was so terrific that it did not rend or crack metal or stone
which it struck. It overcame the chemical forces by which the substance
was held together and reduced them to gas and powder.

And now, after this somewhat formal but necessary description of the
most destructive fighting-