The Outward Urge by John Wyndham

better views and impressions of them. In appearance they were simple. Take an egg, pull it out to double its length, and that is the shape of the body. Put long axles through it near the ends, and fasten tall, wide-tyred wheels on them - tall enough to give it a good ground clearance. Mount the wheels so that they have a hundred and eighty degrees of traverse - that is, so that the treads can be turned parallel with the lines of the axle, whether the wheels themselves are before or behind the axle. And you have this machine. It can move in any direction - or spin in one spot, if you want it to. Not, perhaps, very difficult once you have thought of the idea. Give it a motor in each wheel, and an electronic control to keep it from hitting obstacles. That is not very difficult, either.
    ‘What is not so clear is how you direct it. It was not, very clearly, by dead reckoning. We thought it might be responding to our radio, or to the rotation of our radar scanner, or to the movements of our television pick-up, but we tested all those, and even switched off our screen for some minutes, but the guard outside reported no effect. Nor was it detecting and seeking any of our electric motors; we stopped every one of them for a full minute, without any result. It was just possible that the things were picking up an emanation from our power-pile, but that was well shielded, and we already had decoy radiators to deflect any missiles that might try that. I myself think it probable that they were able to detect, and to respond to, the inevitable slight rise of temperature in the station area. If so, there was nothing we could have done about it.’
    The General shrugged, shook his head, and frowned. He went on:
    ‘What we faced, in essence, was a seeking missile on wheels. Not difficult to construct, though scarcely worth attempting for use in a simple form - too easy a target for the defence. So what those Americans had done, the frightening thing they had done, was to introduce a random element. You see what I mean? They had put in this random stage, and somehow filtered the control through it...
    He thought again for a moment.
    ‘Machines do not live, so they cannot be intelligent. Nevertheless, it is in the nature of machines to be logical. The conception of an illogical machine seems to be a contradiction in terms. If you deliberately produce such a thing, what have you? Something that never existed in nature. Something alien. What you have done is to produce madness without mind. You have made unreason animate, and set it loose. That is a very frightening thing to think about....
    ‘But here, among these not-quite-machines that were scuttering about the crater floor like water-boatmen on a pond, there was a controlling thread of ultimate purpose running through the artificial madness. Their immediate actions were unpredictable, insane, but their final intention was just as sure as the bomb that each was carrying in its metal belly. Think of a maniac, a gibbering idiot, with one single continuing thread of intention - to murder. ...
    ‘That is what those machines were. And they kept on coming with short, or very short, or not so short crazy rushes. They darted and dodged forward, sideways, backwards, obliquely, straight, or in a curve; one never knew which would be next - only that, after a dozen moves, they would be just a little closer.
    ‘Our rocket men opened fire at about five kilos. A sheer waste of course; one could as well have hoped to hit a fly on the wing with a peashooter. Mines might have stopped them - if they did not have detectors - but who would have sanctioned the use of valuable rocket-space to bring mines to the moon? All our men could do was to hope for a lucky shot. Occasionally one of them would be hidden for a moment or two by the burst of an explosion, but it always reappeared out of the dust, dodging as crazily as ever. Our eyes and heads ached with the strain of trying to follow them on the screen, and to