Zak led her to a small apparatus attached to the wire that marked the Null Line. It was a tube of susk cuticle, containing a spring with a stone at one end, much like the one she used to measure weights. Here, of course, the spring was unstretched, and the stone lay beside a mark on the tube that indicated no weight at all.

The end of the tube opposite the stone was attached to the wire by a small loop that allowed it to pivot. Zak flicked the tube and set it spinning, the free end sweeping out a circle while the other remained fixed. "What do you see?"

"The spring is stretched now," Roi observed. "As if the stone had weight."

"Yes." Zak reached over and gave the tube another sharp tap, setting it moving faster. "And now?"

"It's stretched even more. As if the weight had increased."

"Good. Now let's put some numbers to this."

Zak took a sheet of cured skin from his carapace, and had Roi count while the tube spun around, to judge how quickly it made each revolution. Six times, they spun the tube and recorded both the time it took to complete a circle and the weight indicated by the stretching of the spring. A special kind of pointer that could only move one way under pressure from the stone made it possible to read the weight off the scale after the tube was brought to a halt; squeezing the pointer made it narrower and allowed it to be slid back, resetting the weight.

Zak said, "Multiply the weight by the time, and then by the time again."

Roi stared at the skin, as if the answers might simply leap into her mind, but nothing happened. "I can't do that," she admitted. She understood the concept, but when it came to manipulating actual figures she had only been taught how to add and subtract. "None of my teams ever needed multiplication."

"All right, don't worry, I'll teach you later." Zak moved down the list of figures, rapidly scratching in the results. Although the individual times and weights varied greatly, the numbers produced by his calculation—weight by time, by time again—were all similar, all close to two hundred and seventy.

Roi was mystified. "Two hundred and seventy? What does that mean?"

"Nothing. Ignore the particular value, it's just a measure of such things as how fast you count, and how we assign numbers to weights. The important thing is, we always get the same value, however fast or slow the stone is moving. There's a rule here, there's a pattern."

"Not a very simple one," Roi protested.

"Be patient."