The builder nodded rapidly.  “All right.  But why doesn’t the electricity affect the walls themselves?  I thought they were made of steel.”

The engineer glanced through the dead-light at the reddish disk of the Earth, hazy and indistinct at a distance of forty million miles.  “It isn’t steel; it’s a non-magnetic alloy.  Besides, there’s a layer of crystalline sulphur between the alloy and the vacuum space.”

“The vacuum is what keeps out the cold, isn’t it?” Jackson knew, but he asked in order to learn more.

“Keeps out the sun’s heat, too.  The outer shell is pretty blamed hot on that side, just as hot as it is cold on the shady side.”  Smith seated himself beside a huge electrical machine, a rotary converter which he next indicated with a jerk of his thumb.  “But you don’t want to forget that the juice outside is no use to us, the way it is.  We have to change it.

“It’s neither positive nor negative; it’s just neutral.  So we separate it into two parts; and all we have to do, when we want to get away from the earth or any other magnetic-sphere, is to aim a bunch of positive current at the corresponding pole of the planet, or negative current at the other pole.  Like poles repel, you know.”

“Listens easy,” commented Jackson.  “Too easy.”

“Well, it isn’t exactly as simple as all that.  Takes a lot of apparatus, all told,” and the engineer looked about the room, his glance resting fondly on his beloved machinery.

The big room, fifty feet square, was almost filled with machines; some reached nearly to the ceiling, the same distance above.  In fact, the interior of the “cube,” as that form of sky-car was known, had very little waste space.  The living quarters of the four men who occupied it had to be fitted in wherever there happened to be room.  The architect’s own berth was sandwiched in between two huge dynamos.

He was thinking hard.  “I see now why you have such a lot of adjustments for those tunnels,” meaning the six square tubes which opened into the ether through the six walls of the room.  “You’ve got to point the juice pretty accurately.”

“I should say so.”  Smith led the way to a window, and the two shaded their eyes from the lights within while they gazed at the ashy glow of Mercury, toward which they were traveling.  “I’ve got to adjust the current so as to point exactly toward his northern half.”  Smith might have added that a continual stream of repelling current was still directed toward the earth, and another toward the sun, away over to their right; both to prevent being drawn off their course.

“And how fast are we going?”

“Four or five times as fast as mother earth:  between eighty and ninety miles per second.  It’s easy to get up speed out here, of course, where there’s no air resistance.”

Another voice broke in.  The geologist had finished his potatoes, and a savory smell was already issuing from the frying pan.  Years spent in the wilderness had made the geologist a good cook, and doubly welcome as a member of the expedition.