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.