Mammoth by John Varley

protons—”
    “I understand isotopes. Go on, Matt. If you get beyond me, I’ll let you know.”
    “Sorry, Howard, I keep forgetting…”
    That I’m smarter than
you
are, except in the really rarefied realms of math
, Howard thought. It grated on him, but he kept quiet about it because he needed Matt. Matt was a professor, after all, used to lecturing. And he’d probably been doing a lot of it lately, on his daily dates with Susan Morgan. Was there love in the air?
    “Okay. Different isotopes have different weights, per atom. The ratio of isotopes found naturally is fairly standard; a lot of them decay into something else. Almost all the single-element spheres are what you’d expect, not some exotic variation. You follow?”
    Howard nodded.
    “But a few were a little odd. Take osmium. Atomic number, 76. Atomic weight, 190 and change. Seven stable isotopes, six radioactive ones, but with half-lives so short there’d be almost none in a normal sample. Commonest isotope, Os-192. Seventy-six protons and one hundred sixteen neutrons. A bit over forty percent of osmium ought to be Os-192. But our little ball only has thirty-five percent. To compensate, there’s more Os-188 than there should be.”
    “Is it a radioactive decay thing?” Howard asked. “One form of osmium emits an alpha particle—”
    “No, no. Osmium decays into rhenium and iridium, a little tungsten later on. Those are all there, in trace amounts, what we’d expect. No, somebody, the builder, made sure the osmium ball had a different isotopic ratio from normal. So we
have
to duplicate that ratio, because it’s so weird it just
has
to be something important.” He stopped, and looked at Howard for a moment. “Don’t you think?”
    Howard laughed. “That’s what I’m paying you the big bucks for. If you think it’s important, I will, too.”
    Matt took one of the spheres of the odd osmium, shiny as mercury, and slipped it into a little metal rack, then snapped it into place. He stood back and regarded it.
    “There we are,” he said. “The Howard Christian Time Machine, Mark One.”
    Howard looked surprised.
    “You mean it’s finished?”
    “It’s assembled. What comes next is anybody’s guess.”
    “I’m paying you to guess.”
    Matt sighed. “Yes, you are. But I don’t have the foggiest idea what to do at this point. I can manipulate it…” He flipped the assembly of marbles onto its side and slid a row of them to the left, then pushed another row back. Several other slides, and it was back together, ten by twelve by twenty, but the marbles were in a slightly different arrangement.
    “This way leads to madness. The permutations are damn near infinite. There’s a little circuit board in there, identical to the one in the original machine. I went out and bought them at Radio Shack, off the rack. It has a small IC chip, a processor, this and that, none of which seems to be connected in a very logical way. I’ll experiment with that. It has two batteries and two lights. What it doesn’t have is an on/off switch that I can see, any way of setting your destination in time or space, or a user’s manual.”
    Howard clapped him on the shoulder. “You’ll figure it out.”
    “Well, I intend to spend the next year trying, anyway.”
    “Maybe you should just bash it. That usually works.” He thumped the case with his fist. Nothing happened. He shrugged, turned, and started back to his car.
    “Oh, by the way…,” Matt said. Howard stopped and turned back toward him. “If you’re going to take some of themarbles with you, it would be a lot easier on us if you’d let us know which ones you’re taking. I mean, so we can restock.”
    Howard stared at him for a long moment.
    “I don’t know what you’re talking about,” he said.
    “Oh, it’s not a problem, I mean, not a bad one. And you can do what you want, I know that, you already own all the marbles…so to speak.” Matt laughed, but it sounded a little hollow, even to