from the
vessels upon whose tracks they are."

"'Nobody Holme' is right!" exclaimed Seaton, tapping his temple with an
admonitory forefinger. "'Sright, ace--I thought maybe I'd quit using my
head for nothing but a hatrack now, but I guess that's all it's good
for, yet. Thanks a lot for the idea--that gives me something I can get
my teeth into, and now that Rovol's got a problem to work on for the
next century or so, everybody's happy."

"How does that help matters?" asked Crane in wonder. "Of course it is
not surprising that no lines of force were visible, but I thought that
your detectors screens would have found them if any such guiding beams
had been present."

"The ordinary bands, if of sufficient power, yes. But there are many
possible tracer rays not reactive to a screen such as I was using. It
was very light and weak, designed for terrific velocity and for
instantaneous automatic arrest when in contact with the enormous
forces of a power bar. It wouldn't react at all to the minute energy
of the kind of beams they'd be most likely to use for that work.
Caslor's certainly right. They're steering their torpedoes with tracer
rays of almost infinitesimal power, amplified in the torpedoes
themselves--that's the way I'd do it myself. It may take a little while
to rig up the apparatus, but we'll get it--and then we'll run those
birds ragged--so fast that their ankles'll catch fire--and won't need
the fourth-dimensional correction after all."

* * * * *

When the bell announced the beginning of the following period of labor,
Seaton and his co-workers were in the Area of Experiment waiting, and
the work was soon under way.

"How are you going about this, Dick?" asked Crane.

"Going to examine the nose of one of those torpedoes first, and see what
it actually works on. Then build me a tracer detector that'll pick it up
at high velocity. Beats the band, doesn't it, that neither Rovol nor I,
who should have thought of it first, ever did see anything as plain as
that? That those things are following a ray?"

"That is easily explained, and is no more than natural. Both of you were
not only devoting all your thoughts to the curvature of space, but were
also too close to the problem--like the man in the woods, who cannot see
the forest because of the trees."

"Yeah, may be something in that, too. Plain enough, when Caslor showed
it to us," said Seaton.

While he was talking, Seaton had