preventive of the disaster overtaking the ship from the mine on
Orede. It was essentially a tank of liquid oxygen, packed in the space
from which stores had been taken away. When the ship's air-supply was
pumped past it, first moisture and then CO2 froze out. Then the air
flowed over the liquefied oxygen at a rate to replace the CO2 with more
useful breathing material. Then the moisture was restored to the air as
it warmed again. For so long as the oxygen lasted, fresh air for any
number of men could be kept purified and breathable. The Med Ship's
normal equipment could take care of no more than ten. But with this it
could journey to Weald with almost any complement on board.

Maril stayed on Dara when the Med Ship left. Murgatroyd protested
shrilly when he discovered her about to be closed out by the closing
lock-door.

"_Chee!_" he said indignantly. "_Chee! Chee!_"

"No," said Calhoun, "we'll be crowded enough anyhow. We'll see her
later."

He nodded to one of the first four student pilots, and he crisply made
contact with the landing-grid office. He very efficiently supervised as
the grid took the ship up. The other three of the four first-trained men
explained every move to sub-classes assigned to each. Calhoun moved
about, listening and making certain that the instruction was up to
standard.

He felt queer, acting as the supervisor of an educational institution in
space. He did not like it. There were twenty-four men beside himself
crowded into the Med Ship's small interior. They got in each other's
way. They trampled on each other. There was always somebody eating, and
always somebody sleeping, and there was no need whatever for the
background tape to keep the ship from being intolerably quiet. But the
air-system worked well enough, except once when the reheater unit quit
and the air inside the ship went down below freezing before the trouble
could be found and corrected.

The journey to Weald, this time, took seven days because of the training
program in effect. Calhoun bit his nails over the delay. But it was
necessary for each of the students to make his own line-ups on Weald's
sun, and compute distances, and for each of them to practise
maneuverings that would presently be called for. Calhoun hoped
desperately that preparations for active warfare--or massacre--did not
move fast on Weald. He believed, however, that in the absence of direct
news from Dara, Wealdian officials would take the normal course