n, so to speak, at right angles with that of the
orbit, or along its radius, straight outward from the Sun, forty odd
millions of miles in the same time. If I succeeded in this, I should
reach the orbit of Mars at the point and at the moment of opposition,
and should attain Mars himself. But in this I might fail, and I should
then find myself under the sole influence of the Sun's attraction;
able indeed to resist it, able gradually to steer in any direction
away from it, but hardly able to overtake a planet that should lie far
out of my line of advance or retreat, while moving at full speed away
from me. In order to secure a chance of retreat, it was desirable as
long as possible to keep the Earth between the Astronaut and the Sun;
while steering for that point in space where Mars would lie at the
moment when, as seen from the centre of the Earth, he would be most
nearly opposite the Sun,--would cross the meridian at midnight. It was
by these considerations that the course I henceforward steered was
determined. By a very simple calculation, based on the familiar
principle of the parallelogram of forces, I gave to the apergic
current a force and direction equivalent to a daily motion of about
750,000 miles in the orbital, and rather more than a million in the
radial line. I need hardly observe that it would not be to the apergic
current alone, but to a combination of that current with the orbital
impulse received at first from the Earth, that my progress and course
would be due. The latter was the stronger influence; the former only
was under my control, but it would suffice to determine, as I might
from time to time desire, the resultant of the combination. The only
obvious risk of failure lay in the chance that, my calculations
failing or being upset, I might reach the desired point too soon or
too late. In either case, I should be dangerously far from Mars,
beyond his orbit or within it, at the time when I should come into a
line with him and the Sun; or, again, putting the same mischance in
another form, behind him or before him when I attained his orbit. But
I trusted to daily observation of his position, and verification of my
"dead reckoning" thereby, to find out any such danger in time to avert
it.

The displacement of the Earth on the Sun's face proved it to be
necessary that the apergic current should be directed against the
latter in order to govern my course as I desired, and to recover the
ground I had lost