the Earth, and was falling behind her.  Had I used the apergy only to drive me directly outward from the Sun, I should move under the impulse derived from the Earth about 1,600,000 miles a day, or 72 millions of miles in forty-five days, in the direction common to the two planets.  The effect of the constantly widening orbit would be much as if the whole motion took place on one midway between those of the Earth and Mars, say 120 millions of miles from the Sun.  The arc described on this orbit would be equivalent to 86 millions of miles on that of Mars.  The entire arc of his orbit between the point opposite to that occupied by the Earth when I started and the point of opposition—­the entire distance I had to gain as measured along his path—­was about 116 millions of miles; so that, trusting to the terrestrial impulse alone, I should be some 30 millions behindhand at the critical moment.  The apergic force must make up for this loss of ground, while driving me in a direction, 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.