against regular ships. They have to let it shoot
star-bullets in training, or it'd get hot and bothered in a real fight
when its guns went off."

The lower jet streaked skyward once more. Sparks sped from the
formation. They flared through emptiness where the Mahon jet had been
but now was not. It scuttled abruptly to one side as concerted streams
of sparks converged. They missed. It darted into zestful, exuberant
maneuverings, remarkably like a dog dashing madly here and there in pure
high spirits. The formation of planes attacked it resolutely.

Suddenly the lone jet plunged into the midst of the formation, there
were coruscations of little shooting stars, and one-two-three planes
disgustedly descended to lower levels as out of action. Then the single
ship shot upward, seemed eagerly to shake itself, plunged back--and the
last ships tried wildly to escape, but each in turn was technically shot
down.

The Mahon jet headed back for its own tiny airfield. Somehow, it looked
as if, had it been a dog, it would be wagging its tail and panting
happily.

"That one ship," said Lecky blankly, "it defeated the rest?"

"It's got a lot of experience," said the sergeant. "You can't beat
experience."

He led the way into Communications Center. In the room where Betsy
stood, Howell and Graves had been drawing diagrams at each other to the
point of obstinacy.

"But don't you see?" insisted Howell angrily. "There can be no source
other than a future time! You can't send short waves through
three-dimensional space to a given spot and not have them interceptible
between. Anyhow, the Compubs wouldn't work it this way! They wouldn't
put us on guard! And an extra-terrestrial wouldn't pretend to be a human
if he honestly wanted to warn us of danger! He'd tell us the truth!
Physically and logically it's impossible for it to be anything but what
it claims to be!"

Graves said doggedly:

"But a broadcast originating in the future is impossible!"

"Nothing," snapped Howell, "that a man can imagine is impossible!"

"Then imagine for me," said Graves, "that in 2180 they read in the
history books about a terrible danger to the human race back in 1972,
which was averted by a warning they sent us. Then, from their
history-books, which we wrote for them, they learn how to make a
transmitter to broadcast back to us. Then they tell us how to make a
transmitter to broadcast ahead to them. They don't invent the
transmitter. We tell them ho