, but its almost indefinite powers of
expansion necessitate vast tank room. Even in this thin air the
lift-shunts are busy taking out one-third of its normal lift, and still
"162" must be checked by an occasional downdraw of the rudder or our
flight would become a climb to the stars. Captain Purnall prefers an
overlifted to an underlifted ship; but no two captains trim ship alike.
"When _I_ take the bridge," says Captain Hodgson, "you'll see me shunt
forty per cent. of the lift out of the gas and run her on the upper
rudder. With a swoop upwards instead of a swoop downwards, _as_ you say.
Either way will do. It's only habit. Watch our dip-dial! Tim fetches her
down once every thirty knots as regularly as breathing."

So is it shown on the dip-dial. For five or six minutes the arrow creeps
from 6,700 to 7,300. There is the faint "szgee" of the rudder, and back
slides the arrow to 6,500 on a falling slant of ten or fifteen knots.

"In heavy weather you jockey her with the screws as well," says Captain
Hodgson, and, unclipping the jointed bar which divides the engine-room
from the bare deck, he leads me on to the floor.

Here we find Fleury's Paradox of the Bulkheaded Vacuum--which we accept
now without thought--literally in full blast. The three engines are
H. T. &. T. assisted-vacuo Fleury turbines running from 3,000 to the
Limit--that is to say, up to the point when the blades make the air
"bell"--cut out a vacuum for themselves precisely as over-driven marine
propellers used to do. "162's" Limit is low on account of the small size
of her nine screws, which, though handier than the old colloid
Thelussons, "bell" sooner. The midships engine, generally used as a
reinforce, is not running; so the port and starboard turbine
vacuum-chambers draw direct into the return-mains.

The turbines whistle reflectively. From the low-arched expansion-tanks
on either side the valves descend pillarwise to the turbine-chests, and
thence the obedient gas whirls through the spirals of blades with a
force that would whip the teeth out of a power-saw. Behind, is its own
pressure held in leash or spurred on by the lift-shunts; before it, the
vacuum where Fleury's Ray dances in violet-green bands and whirled
turbillions of flame. The jointed U-tubes of the vacuum-chamber are
pressure-tempered colloid (no glass would endure the strain for an
instant) and a junior engineer with tinted spectacles watches the Ray
intently. It is the very heart o