hy--what in the name of the Planets?" asked McLaurin. "It didn't kill
the men in the forts--why does it kill the men in the ships, when the
ships are protected?"

"The protection kills them."

"But--but they had the protective oscillations on all the way out!"
protested the Commander.

"Think how it works though. Think, man. The enemy's field is an
electric-field oscillation. We combat it by setting up a similar
oscillating field in the metal of the hull ourselves. Because the metal
conducts the strains, they meet, and oppose. It is not a shield--a
shield is impossible, as I have said, because of energy concentration
factors. If their beam carried a hundred thousand horsepower in a
ten-foot square beam, in every ten square feet of our shield, we'd have
to have one hundred thousand horsepower. In other words, hundreds of
times as much energy would be needed in the shield, as they used in
their beam. We can't afford that. We had to let the beams oppose our
oscillations in the metal, where, because the metal conducts, they meet
on an equal basis. But--when two oscillations of slightly different
frequency meet, what is the result?"

"In this case, a heterodyne frequency of a lower, and harmless
frequency."

"So I thought. I was partly right. It does _not_ harm the metal. But it
kills the men. It is super-sonic. The terrible, shrill sounds destroy
the cells of the men's bodies. Then, when their dead hands release the
controls, the automatic switches blow up the ship."

"God! We stop one menace--and it is like the Hydra. For every head we
lop off, two spring up."

"Ah--but they are lesser heads. Look, what is the fundamental difference
between sound and light?"

"One is a vibration of matter and the--ah--eliminate the material
contact!"

"Exactly! All we need to do is to let the ships operate airless, the men
in space suits. Then the air cannot carry the sounds to them. And by
putting special damping materials in their suits, we can stop the
vibrations that would reach them through their feet and hands. Another
six-man ship must go out--but this ship will come back!"

And with the order for another experimental ship, went the orders for
commercial supplies of this new apparatus. Every IP ship must be
equipped to resist it.

Buck Kendall sailed on the six-man scout that went out this time. Again
they swooped once at Phobos, again Miran scout-ships crumbled under the
attack of the vicious UV beams. The Mirans wer