one-half inch in diameter, mounted in tubular trays of
reflective material which took up sufficient space to make each rod
occupy two inches of the circumference of the tube on which it was
mounted.

These ruby rods were the heart of the power system, converting the
random wave fronts of noncoherent light received from the mirror into
a tremendous beam of coherent infrared energy which could be bundled
in such a pattern as to reach Earth's surface in a focal point
adjustable from here to be something between twenty-two feet in
diameter to approximately one mile in diameter.

[Illustration]

The banks of rods were so arranged that each of the one hundred
sections comprising the three thousand feet of receptive surface at
the focus of the mirror formed a concentric circle of energy beams;
each circle becoming progressively smaller in diameter, so that the
energy combined into one hundred concentric circles, one within the
other, as it left the rods; but these circles were capable of the
necessary focusing that could bring them all together into a single
small point near Earth's surface.

* * * * *

The beam leaving the rods represented three hundred seventy-five
million watts of energy, tightly packaged for delivery to Earth. But
this was only a small fraction of the solar energy arriving at the big
mirror.

The remainder, the loss, must be dumped by the black surface at the
back; and to account for the loss in the rods themselves, to prevent
their instantaneous slagging into useless globules of aluminum oxide,
their excess loss energy must also be dumped.

A cooling bath of liquid nitrogen therefore circulated over each rod
and brought the excess heat to the rear of the big lens, where it,
too, could be dumped into the blackness of space beyond.

For all its size and complexity, Hot Rod was only a trifle over six
per cent efficient; but that six per cent of efficiency arriving on
Earth would be highly welcome to supplement the power sources that
statistics said were being rapidly depleted.

The spherical shape of the mirror itself, one of the easiest possible
structures to erect in space, had dictated the placement of the rods
through its center since there was no single focal point for the
entire mirror surface.

But it had also added a complication. From this position, the rods
could have been designed to fire either straight forward or straight
back.

However, due to the