Echoes rolled between the hillsides, but they were not very loud and they soon ended.  Lockley guessed automatically at half a dozen possible causes for the small rock-slide, but he did not think at all of an unperceived temblor from a shock like high explosives going off thirty miles away.

Eight minutes later he heard a deep-toned roaring noise to the northeast.  It was unbelievably low-pitched.  It rolled and reverberated beyond the horizon.  The detonation of a hundred tons of high explosives or an equivalent impact can be heard for thirty miles, but at that distance it doesn’t sound much like an explosion.

He finished his breakfast without enjoyment.  By that time well over three-quarters of the Air Force on the Pacific Coast was airborne and more planes shot skyward instant after instant.  Inevitably the multiplied air traffic was noted by civilians.  Reporters began to telephone airbases to ask whether a practice alert was on, or something more serious.

Such questions were natural, these days.  All the world had the jitters.  To the ordinary observer, the prospects looked bad for everything but disaster.  There was a crisis in the United Nations, which had been reorganized once and might need to be shuffled again.  There was a dispute between the United States and Russia over satellites recently placed in orbit.  They were suspected of carrying fusion bombs ready to dive at selected targets on signal.  The Russians accused the Americans, and the Americans accused the Russians, and both may have been right.

The world had been so edgy for so long that there were fallout shelters from Chillicothe, Ohio, to Singapore, Malaya, and back again.  There were permanent trouble spots at various places where practically anything was likely to happen at any instant.  The people of every nation were jumpy.  There was constant pressure on governments and on political parties so that all governments looked shaky and all parties helpless.  Nobody could look forward to a peaceful old age, and most hardly hoped to reach middle age.  The arrival of an object from outer space was nicely calculated to blow the emotional fuses of whole populations.

But Lockley ate his breakfast without premonitions.  Breezes blew and from every airbase along the coast fighting planes shot into the air and into formations designed to intercept anything that flew on wings or to launch atom-headed rockets at anything their radars could detect that didn’t.

At eight-twenty, Lockley went to the electronic base line instrument which he was to use this morning.  It was a modification of the devices used to clock artificial satellites in their orbits and measure their distance within inches from hundreds of miles away.  The purpose was to make a really accurate map of the park.  There were other instruments in other line-of-sight positions, very far away.  Lockley’s schedule called for them to measure their distances from each