"Weight" is synonymous with the gravity of the Earth. The denser a celestial body, the greater its gravity. The larger such a body is, the more strongly it will attract its objects. The smaller the body (if it has the same density), the smaller its force of attraction. Thus a man weighing eighty kilograms on a _spring_ scale on the earth, would weigh but thirty kilograms on the planet Mars. On the sun, however, he would weigh 2232 kilograms. Inside of a space flyer, which had an infinitely small gravity, objects weighed practically nothing. They were heaviest near the walls of the machine, but in the exact center of the flyer, _all objects lost their weight entirely_. Thus any object, regardless of its earthly weight, _hung freely suspended in the center of the space flyer_. It could not move up or down, of its own accord, but hung stationary, motionless, like a balloon in the air.[9] The occupant of a space flyer, having no weight, moved around with astonishing ease. He almost floated around in the machine. There was no physical labor. The biggest table was no heavier than a match. The passenger in a flyer could perform an incredible amount of work without tiring and without effort. He could walk up the walls or walk "upside down" on the ceiling without danger of falling, as there is no "up" or "down" in outer space. _Sleep was practically impossible._ There being nothing to tire the occupant, _sleep is unnecessary_. Dozing off is all he can do, and that could never last long, except after strenuous mental work. As long as a space flyer was not too far distant from the sun (within the orbit of Mars, at least), little artificial heat was needed. The sun heated one-half of the flyer's shell to a fierce heat, but the side turned away from the sun was exposed to the terrible stellar cold (absolute zero) and a fairly comfortable temperature was the result. The air supply was manufactured by chemical means on board, but very little was needed, as the original supply taken from the earth is used over and over by altering the carbonic acid gas by means of automatic generators. It was of course of the utmost importance that no port-hole or doors leading to the outside be opened. The air would have rushed from the flyer instantly, resulting in a perfect vacuum inside of the flyer, and instant death to all living organisms. As the flyer moved away from a celestial body, the less the mechanical energy needed to propel it. There were of course exceptions. Thus between every two celestial bodies a point will be found where the attraction that one body exerts on the other is zero. If the flyer were brought to this point its gyroscopes could be at rest, as the machine would not be attracted by either body. It would "hang" between the two just as an iron ball hangs between two powerful magnets if carefully balanced. Give it the slightest push, however, and the ball will fly to either of the magnets.