they were actually fired to give the car a fillip when it reached the dead-point on its way back to the earth."

I. "Even in a vacuum, where an ordinary propeller could not act, the bullet may become a prime mover, and co-operate with the gun. A rocket can burn without an atmosphere, and the recoil of the rushing fumes will impel the car onwards."

G. "Do you think a rocket would have sufficient power to be of any service?"

I. "Ten or twelve large rockets, capable of exerting a united back pressure of one and a half tons during five or six minutes on a car of that weight at the earth's surface, would give it in free space a velocity of two miles a second, which, of course, would not be lost by friction."

G. "So that it would not be absolutely necessary to give the projectile an initial velocity of five miles a second."

I. "No; and, besides, we are not solely dependent on the rocket. A jet of gas, at a very high pressure, escaping from an orifice into the vacuum or ether, would give us a very high propelling force. By compressing air, oxygen, or coal-gas (useful otherwise) in iron cylinders with closed vents, which could be opened, we should have a store of energy serviceable at any time to drive the car. In this way a pressure or thrust of several tons on the square inch might be applied to the car as long as we had gas to push it forwards."

G. "Certainly, and by applying the pressure, whether from the rocket or the gas, to the front and sides, as well as to the rear of the car, you would be able to regulate the speed, and direct the car wherever you wanted to go."

I. "Moreover, beyond the range of gravitation, we could steer and travel by pumping out the respired air, or occasionally projecting a pebble from the car through a stuffing box in the wall, or else by firing a shot from a pistol."

G. "You might even have a battery of machine guns on board, and decimate the hosts of heaven."

I. "Our bullets would fly straight enough, anyhow, and I suppose they would hit something in course of time."

G. "If they struck the earth they would be solemnly registered as falling stars."

I. "Certainly they would be burnt up in passing through the atmosphere of a planet and do no harm to its inhabitants."

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