The quaddies arranged themselves around the display in a spherical shell of attentiveness, automatically extending helping hands to neighbors to absorb and trade momentum so that all achieved a tolerable hover. Dr. Yei, sitting in—if you could call it that—floated unobtrusively in the background. Monitoring him for his political purity, Leo supposed, not that it mattered. He did not propose to alter his lecture one jot for her presence.

Leo rotated the image so that each student could see it from every angle. "Now let's magnify this part. You see the deep-V cross section from the high-energy-density beam, familiar from your basic welding courses, right? Note the small round porosities here . . ." The magnification jumped again. "Would you say this weld is defective or not?" He almost added, Raise your hand, before realizing what a particularly unintelligible directive that was here. Several of the red-clad students solved the dilemma for him by crossing their upper arms formally across their chests instead, looking properly hesitant. Leo nodded toward Tony.

"Those are gas bubbles, aren't they, sir? It must be defective."

Leo smiled thanks for the desired straight line. "They are indeed gas porosities. Oddly enough, though, when we crunch the numbers through, they do not appear to be defects. Let us run the computer scan down this length, with an eye to the digital read-out. As you see"—the numbers flickered at a corner of the display as the cross-section moved dizzyingly—"at no point do more than two porosities appear in a cross-section, and at all points the voids occupy less than five percent of the section. Also, spherical cavities like these are the least damaging of all potential shapes of discontinuities, the least likely to propagate cracks in service. A noncritical defect is called a discontinuity." Leo paused politely while two dozen heads bent in unison to highlight this pleasingly unambiguous fact on the autotranscription of their light boards, braced between lower hands for a portable recording surface. "When I add that this weld was in a fairly low-pressure liquid storage tank, and not, for example, in a thruster propulsion chamber with its massively greater stresses, the slipperiness of this definition becomes clearer. For in a thruster the particular degree of defect that shows up here would have been critical.

"Now." He switched the holovid display to one in red light. "This is a holovid of the same weld from data bits mapped by an ultrasonic pulse reflective scan. Looks quite different, doesn't it? Can anyone identify this discontinuity?" He zoomed in on a bright area.

Several sets of arms crossed again. Leo nodded toward another student, a striking boy with aquiline nose, brilliant black eyes, wiry muscles, and dark mahogany skin contrasting elegantly with his red T-shirt and shorts. "Yes, Pramod?"

"It's an unbonded lamination."