They walked through long, high corridors, walled with gray metal and flanked by doors opening into compartments of various styles and equipment. Aided by Yaxa's explanations. Neil was not long in visualizing the whole structure as a series of spherical surfaces, one within another, each surface utilized as the floor of a level. Artificial gravity was set up at the core and elevators and sloping runways permitted the garrison to progress from one level to another. "Most of all," said Neil, "I want to view this wonderful mechanism which holds the four parts of your asteroid together." "A trifle, nothing but a trifle," Yaxa replied with a deprecatory gesture. "The principal is a simple magnetic one. The four sections--the fruit slices, I once described them--bring their inner angles together along a common line. That common line is a long, thin cable made of six different kinds of metal, each of the six connected with a special motor at either end. They set up the current among themselves, and the cable acts as the pole of our world." "And if the current was cut off?" "Then the four sections would float apart. But the current will endure as long as the cable is not cut clean in two." "Then where is the center of gravity?" "At the very mid-point of the cable, which is also the center of the asteroid and of each concentric sphere within it." "I would greatly like to see this cable," said Neil again. "That is the only request I cannot grant you," the Martian replied. "It is the most sacred, the most jealously fenced object of all. Every foot is guarded by trusted men, each one sworn to defend it with his last drop of blood. Only the commander of this garrison can be admitted to the tubular compartment which surrounds its central emanator of gravity, or to the shops where the motors run. But don't feel disappointed over such a prohibition. Come, we'll go to a theater and on the way we'll pass as close to the cable as we're likely to get." Sure enough, as they walked down the corridor they came to a juncture of four wide passages. Here was a small concourse, thronged with pedestrians, and in its very center a stout metal pillar rose from the flooring to the roof. Two sentries stood vigilantly on opposite sides of it. "We are now at the point where the four sections meet on this level," Yaxa pointed out. "As you see, the walls are cut well away to allow the passages to cross. That pillar is made of four pieces--the edges of the sections. Enclosed by them is the cable I told you about. The pillar and the cable extend above and below here, from one pole of the asteroid to the other." Neil looked at the arrangement as if fascinated but Yaxa urged him on. They came to the spot where opposite partitions of two adjoining sections came together. There was not enough space to insert a knife-blade, so accurately had the structure been made.