as discharged through the very centre of the Ring, but it could be
swung through a small angle in any direction, and by means of this
slight deflection the horizontal motion of the machine secured. Perhaps
the most interesting feature of the mechanism was that the Ring appeared
to have automatic stability, for the angle of the direction in which the
tractor was pointed was controlled not only by a pair of gyroscopes
which kept the Ring on an even keel, but also by a manometric valve
causing it to fly at a fixed height above the earth's surface. Should it
start to rise, the diminished pressure of the atmosphere operating on
the valve swung the tractor more to one side, and the horizontal
acceleration was thus increased at the expense of the vertical.]

But the more he learned about the mechanism of the Ring the greater
became his misgivings about undertaking the return journey alone with
Atterbury through the air. If they were to go, the start must be made
within a few days, for the condenser held its charge but a comparatively
short time, and its energy was necessary for starting the Ring. When
freshly charged it supplied current for the thermic inductor for nearly
three minutes, but the metallic films, deposited on the plates,
dissolved slowly in the fluid, and after three or four days there
remained only enough for a thirty-second run, hardly enough to lift the
Ring from the earth. Once in the air, the downward blast from the
tractor operated a turbine alternator mounted on a skeleton framework at
the centre of the Ring, and the current supplied by this machine enabled
the Ring to continue its flight indefinitely, or until the cylinder of
uranium was completely disintegrated.

Yet to trek back over the route by which he had come appeared to be
equally impossible. There was little likelihood that the two Indians
would return; they were probably already thirty miles on their way back
to the coast. If only he could get word to Thornton or some of those
chaps at Washington they might send a relief expedition! But a ship
would be weeks in getting to the coast, and how could he live in the
meantime? There were provisions for only a few days in the Ring, and the
storehouse in the valley had been wiped out of existence. Only an
aeroplane could do the trick. And then he thought of Burke, his
classmate--Burke who had devoted his life to heavier-than-air machines,
and who, since his memorable flight across the Atlantic in the _S