"What's "moss", Grandma Hazell?" Lowell demanded.

"Huh? It's... well, it's what rolling stones don't gather."

Roger snapped his fingers. "Hazel, you've just named the ship."

"Eh? Come again."

"The Rolling Stones. No, the Rolling Stone."

Dr. Stone glanced up. "I like that, Roger."

"Meade?"

"Sounds good, Daddy."

"Hazel?"

"This is one of your brighter days, son."

"Stripped of the implied insult, I take it that means "yes.""

"I don't like it," objected Pollux. "Castor and I plan to gather quite a bit of moss."

"It's four to three, even if you get Buster to go along with you and your accomplice. Overruled. The Roiling Stone it is."

Despite their great sizes and tremendous power spaceships are surprisingly simple machines. Every technology goes through three stages: first, a crudely simple and quite unsatisfactory gadget; second, an enormously complicated group of gadgets designed to overcome the shortcomings of the original and achieving thereby somewhat satisfactory performance through extremely complex compromise; third, a final stage of smooth simplicity and efficient performance based on correct under-standing of natural laws and proper design therefrom.

In transportation, the ox cart and the rowboat represent the first stage of technology.

The second stage might well be represented by the automobiles of the middle twentieth century just before the open­ing of interplanetary travel. These unbelievable museum pieces were for the time fast, sleek and powerful -. but inside their skins were assembled a preposterous collection of mechanical buffoonery. The prime mover for such a juggernaut might have rested in one's lap; the rest of the mad assembly consisted of afterthoughts intended to correct the uncorrectable, to repair the original basic mistake in design - for automobiles and even the early aeroplanes were 'powered' (if one may call it that) by 'reciprocating engines."

A reciprocating engine was a collection of miniature heat engines using (in a basically inefficient cycle) a small percentage of an exothermic chemical reaction, a reaction which was started and stopped every split second. Much of the heat was intentionally thrown away into a 'water jacket' or 'cooling system," then wasted into the atmosphere through a heat exchanger.

What little was left caused blocks of metal to thump foolishly back-and-forth (hence the name 'reciprocating') and thence through a linkage to cause a shaft and flywheel to spin around. The flywheel (believe it if you can) had no gyroscopic function; it was used to store kinetic energy in a futile attempt to cover up the sins of reciprocation. The shaft at long last caused wheels to turn and thereby propelled this pile of junk over the countryside.

The prime mover was used only to accelerate and to overcome 'friction' - a concept then in much wider engineering use. To decelerate, stop, or turn the heroic human operator used their own muscle power, multiplied precariously through a series of levers.

Despite the name 'automobile' these vehicles had no auto­control circuits; control, such as it was, was exercised second by second for hours on end by a human being peering out through a small pane of dirty silica glass, and judging unassisted and often disastrously his own motion and those of other objects. In almost all cases the operator had no notion of the kinetic energy stored in his missile and could not have written the basic equation. Newton's Laws of Motion were to him mysteries as profound as the meaning of the universe.

Nevertheless millions of these mechanical jokes swarmed over our home planet, dodging each other by inches or failing to dodge. None of them ever worked right; by their nature they could not work right; and they were constantly getting out of order. Their operators were usually mightily pleased when they worked at all. When they did not, which was every few hundred miles (hundred, not hundred thousand) they hired a member of a social class of arcane specialists to make inadequate and always expensive temporary repairs.

Despite their mad shortcomings, these 'automobiles' were the most characteristic form of wealth and the most cherished possessions of their time. Three whole generations were slaves to them.

The Rolling Stone was the third stage of technology. Her power plant was nearly 100% efficient, and, save for her gyro-scopes, she contained almost no moving parts - the power plant used no moving parts at all; a rocket engine is the simplest of all possible heat engines. Castor and Pollux might have found themselves baffled by the legendary Model-T Ford automobile, but the Roiling Stone was not nearly that complex, she was merely much larger. Many of the fittings they had to handle were very massive, but the Moon's one-sixth gravity was an enormous advantage; only occasionally did they have to resort to handling equipment.

Having to wear a vacuum suit while doing mechanic's work was a handicap but they were not conscious of it. They had worn space suits whenever they were outside the pressurised underground city since before they could remember; they worked in them and wore them without thinking about them, as their grandfather had worn overalls. They conducted the entire overhaul without pressurising the ship because it was such a nuisance to have to be forever cycling an airlock, dress­ing and undressing, whenever they wanted anything outside the ship.

An IBM company representative from the city installed the new ballistic computer and ran it in, but after he had gone the boys took it apart and checked it throughout themselves, being darkly suspicious of any up-check given by a manu­facturer's employee. The ballistic computer of a space ship has to be right; without perfect performance from it a ship is a mad robot, certain to crash and kill its passengers. The new computer was of the standard 'I-tell-you-three-times' variety, a triple brain each third of which was capable of solving the whole problem; if one triplet failed, the other two would out-vote it and cut it off from action, permitting thereby at least one perfect landing and a chance to correct the failure.

The twins made personally sure that the multiple brain was sane in all its three lobes, then, to their disgust, their father and grandmother checked everything that they had done.

The last casting had been x-rayed, the last metallurgical report had been received from the spaceport laboratories, the last piece of tubing had been reinstalled and pressure tested; it was time to move the Rolling Stone from Dan Ekizian's lot to the port, where a technician of the Atomic Energy Com­mission - a grease monkey with a Ph.D - would install and seal the radioactive bricks which fired her 'boiler." There, too, she would take on supplies and reactive mass, stablised mon-atomic hydrogen; in a pinch the Rolling Stone could eat anything, but she performed best on 'single-H."

The night before the ship was to be towed to the spaceport the twins tackled their father on a subject dear to their hearts - money. Castor made an indirect approach. "See here, Dad, we want to talk with you seriously."

"So? Wait till I phone my lawyer."

"Aw, Dad! Look, we just want to know whether or not you've made up your mind where we are going?"

"Eh? What do you care? I've already promised you that it will be some place new to you. We won't go to Earth, nor to Venus, not this trip."

"Yes, but where?"

" I may just close my eyes, set up a prob on the computer by touch, and see what happens. If the prediction takes us close to any rock bigger than the ship, we'll scoot off and have a look at it. That's the way to enjoy travelling."