G.  “I presume you are taking into account the velocity of the planet in its orbit?  That of the earth is 18 miles a second, or a hundred times faster than a rifle bullet; that of Venus, which is nearer the sun, is a few miles more; and that of Mars, which is further from the sun, is rather less.”

I.  “For that reason the more distant planets would be preferable to land on.  Uranus, for instance, has an orbital velocity of four miles a second, and his gravity is about three-fourths that of the earth.  Moreover, his axis lies almost exactly on the plane of the ecliptic, so that we could choose a waiting place on his orbit where the line of his axis lay in the direction of his motion, and simply descend on one of his poles, at which the stationary atmosphere would not whirl the car, and where we might also profit by an ascending current of air.  The attraction of the sun is so slight at the distance of Uranus, that a stone flung out of the car would have no perceptible motion, as it would only fall towards the sun a mere fraction of an inch per second, or some 355 feet an hour; hence, as Dr. Preston has calculated, one ounce of matter ejected from the car towards the sun every five minutes, with a velocity of 880 feet a second, would suffice to keep a car of one and a half tons at rest on the orbit of the planet.  Indeed, the vitiated air, escaping from the car through a small hole by its own pressure, would probably serve the purpose.  Just before the planet came up, and in the nick of time we could fire some rockets, and give the car a velocity of two or three miles a second in the direction of the planet’s motion, so that he would overtake us, with a speed not over great to ensure a safe descent.  Our parachutes would be out, and at the first contact with the atmosphere, the car would probably be blown away; but it would soon acquire the velocity of the planet, and gradually sink downwards to the surface.”

G.  “What puzzles me is how you are to get back to the earth.”

I.  “Whoever goes must take the risk; but if, as appears likely, both Mars and Venus are inhabited by intelligent beings, we should probably be able to construct another cannon and return the way we came.”

G. (smiling).  “Well, I confess the project does not look so impracticable as it did.  After all, travelling in a vacuum seems rather pleasant.  One of these days, I suppose, we astronomers will be packed in bullets and fired into the ether to observe eclipses and comets’ tails.”

I.  “In all that has been said we have confined ourselves to ways and means already known; but science is young, and we shall probably discover new sources of energy.  It may even be possible to dispense with the gun, and travel in a locomotive car.  Lord Kelvin has shown that if Lessage’s hypothesis of gravitation be correct, a crystal or other body may be found which is lighter along one axis than another, and thus we may be able