The Collected Stories of Arthur C. Clarke by Arthur C. Clarke Page B

it keeps going on and off abruptly. Just as if someone’s working a switch.’
    ‘Sounds very mysterious. Ah, here’s Jones. By the look of him I’d say the Welsh Wonder has found something.’
    Another physicist had just hurried into the room, trailing several yards of recording tape behind him. ‘Got it!’ he cried triumphantly. ‘Look!’ He spread the tapes out over the nearest table, collecting some dirty looks from a party of bridge players who were heading toward it.
    ‘This is the magnetic record. I’ve reduced the sensitivity to one of the recorders until it no longer shoots off the paper. You can see exactly what’s happening now. At these points the field starts to rise rapidly to over a thousand times its normal value. It stays that way for a couple of minutes and then drops back to normal—so.’
    With his finger Jones traced the rise and fall of the magnetic field. ‘There are two things to note. The rise isn’t instantaneous but takes just over a second in each case. It seems to be exponential. That’s just what happens, of course, when you switch on the current in an electromagnet. And the fall is just the same while the plateau in between is perfectly flat. The whole thing is obviously artificial.’
    ‘That’s exactly what I said in the first place! But there’s no such magnet on the Observatory.’
    ‘Wait a minute—I haven’t finished yet. You’ll see that the field jumps up at fairly regular intervals and I’ve carefully noted the times at which it’s come on. I’ve had the whole staff going through the tapes of every automatic recorder in the place to see if anything else has happened at the same instants.
    ‘Quite a lot has—nearly all the records show some fluctuations. The cosmic ray intensity, for instance, falls off when the field goes on. I suppose all the primaries are being swept into it so that we don’t receive them. But the oddest of all is the seismograph tape.’
    ‘ Seismograph ! Who ever heard of a magnetic moonquake?’
    ‘That’s what I thought at first, but here it is. Now if you look carefully you can see that each of the little moonquakes arrives just about a minute and a half after the jolt in the magnetic field, which presumably travels at the velocity of light. We know how fast waves travel through the lunar rock—it’s about a mile a second.
    ‘So we are forced to the conclusion that about a hundred miles away someone is switching on the most colossal magnetic field that’s ever been made. It’s so huge it wrecks our instruments, which means that it must run into millions of gauss.
    ‘The earthquake—sorry, moonquake—must be a secondary effect. There’s a lot of magnetic rock round here and I imagine it must get quite a shock when that field goes on. You probably wouldn’t notice the quake even if you were where it started but our seismographs are so sensitive they’ll spot meteors falling anywhere within twenty miles.’
    ‘That’s about the best piece of high-speed research I’ve ever encountered.’
    ‘Thanks, but there’s still more to come. Next I went up to Signals to find if they’d noticed anything. And were they in a rage! All communication has been wrecked by bursts of static at exactly the same instants as our magnetic barrages. What’s more they’d taken bearings on the source—and with my ranges we have it pinpointed exactly. It’s coming from somewhere in the Sea of Rains, about five miles south of Pico.’
    ‘Holy smoke!’ said Wheeler. ‘We might have guessed!’
    The two physicists pounced on him simultaneously. ‘Why did you say that?’
    Remembering his promise Wheeler looked hesitantly at Jamieson, who came to the rescue. ‘We’ve just come back from Pico. There’s a Government research project going on out there. Very hush-hush—you can’t get near the place. It’s a big dome out on the plain, at least twice the size of the Observatory. Must have a lot of stuff in it from what they say.’
    ‘So