hollow nature of the thirty-five hundred foot
laser barrel; the necessity for access to the rods from inside that
barrel; and the placement of the control booth at its outside end, the
firing could only be forward, straight towards the sun on which the
mirror was focused.

But to be useful, the beam must be able to track an ever-moving
target.

This problem had been solved by one of the largest mirror surfaces
that man had ever created--flat to a quarter of a wave-length of
light, and two hundred fifty feet in diameter, the beam director, from
this distance looking as though it were a carelessly tossed
looking-glass from milady's handbag, anchored one diameter forward of
the big power balloon.

For all its size, this director mirror had very little mass.
Originally it had been planned to be made of glass in much the same
manner as Palomar's 200-inch eye. But this plan had been rejected on
the basis of the weight involved.

Instead, its structure was a rigid honeycomb of plastic; surfaced by a
layer of fluorocarbon plastic which had been brought to its final
polish in space, and then carefully aluminized to provide a highly
reflective, extremely flat surface.

This mirror was also cooled by the liquid nitrogen supplied from the
back side of the big mirror. Necessarily so, since even its best
reflectivity still absorbed a sufficient portion of the energy from
the beam it deflected to have rapidly ruined it if it were not
properly cooled.

The several tons of ruby rods in the barrel, with their clear sapphire
coatings, were far more valuable than any gems of any monarch that had
ever lived on Earth. Synthetic though they were, Steve Elbertson, the
project's military commander, knew they had been shipped here at
fantastic cost and were expected to pay for themselves many thousands
of times over in energy delivered.

* * * * *

As yet, the project had had no specific target; nor had it been fully
operational as of midnight yesterday.

But this "morning" for the first time the terrific energy of the laser
beam would be brought to bear on the Greenland ice cap--three hundred
seventy-five million watts of infrared energy adjusted to a
needle-point expected to be twenty-two feet in diameter at Earth's
surface, delivering one million watts per square foot, that should put
a hole a good way through the several thousand feet of glacier there
in its fifteen minutes of operation, possibly e