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 myself and the Sun, for a reason which may not at first be obvious. The moment of opposition is not necessarily that at which Mars is nearest to the Earth, but is sufficiently so for practical calculation. At that moment, according to the received measurement of planetary distances, the two would be more than 40 millions of miles apart. In the meantime the Earth, travelling on an interior or smaller orbit, and also at a greater absolute speed, was gaining on Mars. The Astronaut, moving at the Earth's rate under an impulse derived from the Earth's revolution round the Sun (that due to her rotation on her own axis having been got rid of, as aforesaid), traveller in an orbit constantly widening, so that, while gaining on Mars, I gained on him less than did 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