an instrument which will tell you how fast we
are going?" she asked in astonishment.

"Unfortunately not. We are traveling through no medium which is dense
enough to register on an instrument. Our course is not straight, but is
necessarily an erratic one as we are subject to the gravimetric pull of
all of the celestial bodies. Just now the Earth supplies most of the
pull on us but as soon as we approach the moon, we will tend to fall on
it and frequent sideblasts will be needed to keep us away from it. Once
we get up some speed that is comparable with light, we can measure by
direct comparison, but our speed is too low for that now."

"I saw you lay out your course, but how are we steering?"

"The observer who works on the front instrument keeps a cross hair on a
fixed star. When the curving of the ship deviates us more than five
degrees from our course, a side motor is turned on until we straighten
out again. It is quite a simple matter and I'll take the ship myself
when we near Mars. There is no need to be frightened."

"I'm not frightened," said Lura quickly; "I was just curious. Is there
any danger of hitting a wandering body?"

* * * * *

"Not much in this zone and at this speed. When our speed picks up there
will be a slight danger because the higher our rate of speed, the more
crowded space becomes. If we were going to Jupiter we would have to use
much more caution. The asteroid belt lying between Mars and Jupiter is
really crowded with small bodies but comparatively few are in the zone
between Earth and Mars. That is one thing I figured on when I said that
we would have plenty of time to go to Mars and back before ships could
come from Jupiter. Ships from Jupiter would be able to develop a much
higher speed than we will attain were it not for the asteroid belt. They
will have to travel quite slowly through it, in portions, not over a few
thousand miles per minute, while we are not held down that way. Now that
we are really started, it will be best to set regular watches. I will
assign you as navigator for one watch if you wish."

"I certainly do want to do my share."

"All right, we'll let it go that way. Turgan and I will take the other
two watches until we get there."

"How soon will that be?"

"About seventeen days. Mars happens to be only about forty million
miles away just now. Now I'll set the watches and divide the crew."

A short examination showed D