Roof of the World
I
‘IF you’ll rise early to-morrow, you can talk to Australia,’ said the obliging young engineer who had been my guide through the research laboratories. So I got up before dawn, and we raced the sun seven miles to an isolated hilltop, where I sat before a microphone in a tiny office and talked and listened to Australia.
Neither I nor the man in Sydney had anything personal to say to one another. But his brogue interested me, and he said mine was a new one to him; then Oakland, California, put in a pert remark, and — well, it was exciting to be talking from Schenectady with the other side of the planet and suddenly to have the other side of the continent breeze in. It made the world uncommonly compact — this ‘ seventh-of-asecond earth,’ Dr. A. E. Kennelly has called it, since our modern Puck of the radio’s dancing electrons can girdle the globe seven times in a second.
Presently a question addressed to Oakland got no answer. I repeated the query. Still it was ignored. Sydney tried to help by saying, ‘Oakland, Schenectady is calling you,’ and I heard it repeat the prompting again and again. Oakland heard Sydney, but was unable to hear or be heard by Schenectady, and we concluded our three-cornered conversation lamely by getting Sydney to repeat my question and Oakland’s answer. What had seemed so effortless a minute ago was now a frustration. It was as though some gigantic breath had blown out the ether between the two coast lines of the United States. And it moved on farther west. By 9 A.M. conversation from Sydney was beginning to splutter and shrill, and soon it too faded.
‘Skip distance,’ explained the young engineer, enigmatically.
Radio dispatchers, whose job is to keep in communication with moving airplanes, tell of strange pranks of ‘skip distance,’ of sudden lapses of silence, of minutes when the plane is lost and they can only wait and hope for the best. A few days ago the voice of a pilot flying over Redding, California, was heard in Des Moines, Iowa, 1900 miles distant, though his own dispatcher at Oakland, 300 miles away, could hear nothing of it.
A black cat unexpectedly walked across the stage of a t heatre where four hundred Rotarians had gathered in the early morning for a radio exchange of greetings with fellow optimists around the world. England, Java, and Australia received the American greeting successfully, but the Australians were unable to get a word across to the waiting four hundred. Altogether it was a rather ragged performance, and, of course, blame was put on the black cat. But the engineers, busy with calculations of the skip distance, knew that the real black cat was the sunlight racing westward over the darkened earth.
Dr. Edward O. Hulburt put the matter thus: ‘It’s the sun that’s behind our radio troubles, and also it’s the sun that’s behind our radio success. What the sun does to the earth’s high atmosphere makes radio communication around the world possible — and also makes it frequently a jumble, a puzzle, sometimes a confusing nuisance. Static, fading, and most of the other interruptions that radio is heir to may be traced to influences of the sun — along with a whole magician’s pack of colored rays, flickering curtains, luminous bands, and other mysteries of the upper air.’
Dr. Hulburt is of the Naval Research Laboratory. No organization is more practically interested in radio than the Navy. The most powerful broadcasting stations in the United States are the naval transmitters at Arlington and Annapolis; their time signals have been heard on the Indian Ocean, in Antarctica, Greenland, and on every continent. New naval stations now being planned will put on the air radio waves measuring 25 miles from one wave crest to the next. At the same time, because of the greater carrying ability of short waves, the Navy is a leading experimenter with wave lengths of 80 metres and under. The numerous shore stations and ships at sea provide means for almost continuous tests, and the vital importance of naval communication makes these studies a paramount interest.
These studies all head up at the Naval Research Laboratory, that secluded retreat at Anacostia, D. C., on the outskirts of Washington. Dr. Hulburt’s specialty is light: he is chief of the division of optics in the Laboratory. Since radio waves are a species of light, the technical problems that arise from the Navy’s far-flung use of wireless naturally gravitate to Dr. Hulburt’s division, where he and his technical associate, H. B. Maris, another young doctor of philosophy from Johns Hopkins, have devoted their major efforts during the last five years to radio riddles. These studies led them from the earth to the sun, to the effects of the sun on the earth’s atmosphere, and particularly to that high, rare, attenuated region in the top story of our sky where mysterious electrical currents are continually pulsing, where death-dealing solar rays are filtered out of the beneficent sunshine, and where this seventh-of-a-second earth has its radio roof.
II
The radio roof was discovered by Marconi. On that gusty December day in 1901 when he sat alone in a hospital room on the Newfoundland coast with a pair of head phones clamped over his ears, while outside his men struggled with an enormous kite to support the slender wire aerial, it was a signal from England that Marconi awaited. Scientists had said that wireless waves could not be sent around the earth: they would sheer off into space at tangents from their points of origin. Marconi, letting only a few intimates into his secret, had made the trip from England to Newfoundland for the sole purpose of testing this idea. For an hour he sat like a man in a waxworks, motionless, tense, listening. At half past twelve, noon, a faint staccato quivered in the phones — the three short dots of the letter s repeated over and over again. It was the prearranged signal. He called his men. Nervously, almost violently, he handed the phones to one of them, saying, ‘ Can you hear anything? ’ The instrument was passed to the next man, and to the next. Each in turn heard the feeble click of the code, ‘zip-zip-zip.’ Wireless power had swung its mysterious resonance across the Western Ocean to be heard by a human ear for the first time.
How had it done this, asked Lord Rayleigh, in England. The waves could not travel through the earth; how could they curve around it?
The answer came simultaneously and independently from two electrical engineers, A. E. Kennelly of Harvard and Oliver Heaviside of the Royal Society in England, who suggested that there might be a layer of electrified particles in the upper atmosphere, and such a layer would serve as a reflector of radio waves. Experiments since have demonstrated unquestionably the existence of this peculiar gaseous shell surrounding the earth. Indeed, the strange humors and vagaries of wireless waves cannot be explained without recognizing this invisible roof. We know that the Kennelly-Heaviside layer is there in somewhat the same way that Percival Lowell and W. H. Pickering knew that Pluto was in the sky — by observing its influence on other phenomena.
A radio wave quivers from its sending station at a speed of 186,000 miles a second, traveling upward and outward in all directions until it strikes the inner surface of the radio roof. This acts as a mirror, and reflects the wave back to the ground at exactly the same angle. Between the place of origin and the region at which the reflected wave first reaches the ground is a zone of silence, an area in which the waves simply do not exist. This zone — the skip distance — varies for different sizes of waves. My conversation with the man in Australia was carried on a wave of 31 1/2 metres, and for this wave length at that hour the skip distance was about 500 miles. Such near-by places as Buffalo, New York, and Philadelphia were in zones of silence, though listeners in Chicago, Denver, San Francisco, and Sydney could hear. Other wave lengths have other skip distances. For a 40metre wave the skip is only 200 miles, and in general the distance decreases as the wave length increases. For the long wave lengths used in general broadcasting there is no obvious skip, since the larger waves fairly blanket the whole area within reach of their carrying power.
Radio thus gives some reality to a geographical concept that was laughed out of court many years ago — the idea that the earth is a hollow sphere. It is indeed a series of spheres — the solid or lithosphere of the geologist, the liquid or hydrosphere of the oceanographer, the aerial or atmosphere of the meteorologist, and now this new-found electrosphere of the wireless expert.
Between the hollow electrosphere above and the more substantial globe of rock and soil and water below bounce the radio waves of our broadcasts and communications. The waves glance back to the roof from the ground at each rebound, and thus zigzag their way around the world. Their zigzags describe angles so mathematically orderly that the researchers were able to measure the height of the layer and to compute what size wave would reach a given point.
But it was found that the computations did not hold for all hours and seasons. The skip distance for a 25metre wave is 500 miles at noon, 700 miles just after sunset, 1000 miles at midnight, and 600 miles just after sunrise. These values are for winter; in summer the distances are all changed, mostly shortened.
It was thus discovered that the Kennelly-Heaviside layer alternately lifts and lowers, sometimes dropping within 60 miles of the earth’s surface, at other times rising to a height of 200 miles and more. There are daily liftings and lowerings that keep in step, approximately, with the movement of the sun across the sky. There are seasonal changes that also seem to have, some relation to the solar position. And there are spasmodic changes that frequently synchronize with the occurrence of sunspots and other solar outbursts.
Picture the radio roof, then, as a billowing canopy of electrified particles. Between the solid earth and the inner surface of this ever-changing bubble of electricity our wireless waves must somehow bump their way, sometimes shooting up 60 miles before they strike the reflector, sometimes 100 or 200 miles. No wonder they get mixed up, and shift their zones of silence like the shadows from a wind-blown torch. No wonder Oakland found that it could n’t hear; and it is n’t strange that the Rotarians were disappointed in Australia, black cat or no black cat. To build a science of these eternally fluctuating phenomena, to plumb and survey and chart this fluttering shell of gas, would seem to be a lunatic’s project — but sober, clear-eyed physicists are doing it, gradually building up a body of law, slowly learning the ‘where’ and the ‘what’ and the ‘how’ of the Kennelly-Heaviside layer.
Like a vast circus tent caught in a gale, bulging boisterously in one place, sagging in another, tossed upward and outward or perhaps beaten down by the mighty cosmic forces pelting the far outposts of our world, is this enclosing film of stampeding particles — the last frontier of the planet earth.
And the gale that billows it is a blast from the sun.
III
How does the sun do this? Hulburt and Maris say its ultra-violet light is the responsible critter. These extremely short rays are packed with enormous energy, many times that of visible rays. When they reach the outer fringes of our atmosphere, they smash its atoms and disrupt molecules, colliding with and knocking out electrons which absorb the rays’ energy, and plunge madly into the sea of space like the bedeviled swine of the New Testament story. The number of particles thus electrified (or ionized) is in direct proportion to the volume of ultra-violet light. The more rays there are, the deeper they will be able to penetrate into the atmosphere and the greater will be their wreckage of particles. It is because of this that the height of the Kennelly-Heaviside layer varies between night and day and between summer and winter. The more sunlight, the more atomic wreckage. When the sun is directly overhead, its rays have their best chance to penetrate deep into the atmosphere and build up a strong electrification. It has been found that, the layer is generally thickest and densest shortly after noon, reaching a maximum at about 1 P.M., shrinking to a minimum around 4 A.M.
Within recent months a curious afterdark transformation has been discovered. As the sun sets, the layer not only shrinks and thins, but it splits into two layers. Thus there is over the night side of the earth this shrunken doubled layer of electrification, while over the day side the layer is single and swollen to greater volume. This peculiar dualism makes it difficult for short waves to travel across sunset or sunrise. Often a short wave will penetrate the inner stratum and pass through it to the outer one, there to be reflected back or perhaps to be imprisoned between the two and be absorbed.
At all times the layer is a region of turmoil and stress, and especially in the sunlit hemisphere. On that side, where the solar rays are continually bombarding the atmosphere, neutral air particles flow upward by convection currents, to be electrified when the ultra-violet radiation strikes them, and then to participate in a vast spiraling of the energized particles (ions) toward the magnetic poles of the earth. The negatively charged particles drift westward, the positively charged particles eastward. This effect of the solar ultraviolet constitutes an electric current (of about 8,000,000 horsepower energy) continually flowing around the earth.
This perpetual current is the Kennelly-Heaviside layer. The roof of the world is not only a flapping tent, it is also a river of electricity forever flowing above our blue sky — a torrent of energy coursing around the world like the torrent of electrons surging through the coils of a dynamo.
Just as the energy flow through the coils of a dynamo induces that mysterious force which we label magnetism, so this electrical flow in the high atmosphere has a dynamo effect on the earth. Geophysicists long puzzled over certain discrepancies in the earth’s magnetism. Their analysis showed that most of it arises within the earth, but about 2 per cent could not be accounted for in this way and must therefore be of external origin. Now it is found that the current of the Kennelly-Heaviside layer is of such volume (about 3,000,000 amperes) as will generate a magnetic field exactly equal to the 2 per cent of extra-terrestrial origin.
Does this mean kinship between the radio roof and such an ancient device as the mariner’s compass?
Apparently so; for whenever the sensitive needle of the magnetic observatory dances its inexplicable jig under the influence of those strange invisible tornadoes known as magnetic storms, queer things happen to radio. At these times the radio roof expands to enormous proportions, radio waves get shunted and entangled by the sudden changes in height and density of the layer, and sometimes for hours and even days communication is jumbled, if not impossible.
Thus, on July 7, 1928, the transatlantic short-wave radio suddenly became inoperative. Wireless telephone by short wave from New York to San Francisco was out of commission all day, and transmission from West to East was little better. Certain telegraph lines burned out their protectors, and wire telephone communication between many points was interrupted. That night one of the most brilliant displays of the aurora borealis in recent times flared up and drooped its flickering green curtains over the northern sky; it was seen as far south as Atlanta and Dallas. During these strange contortions, the sensitive compasses in magnetic observatories all over the earth jigged back and forth a simultaneous record of a magnetic storm — one of the most violent in recent years, though silent and imperceptible except to the sensitive nerves of electrical circuits and radio waves.
Solar photographs taken at Mount Wilson and other stations that day showed a flock of sunspots passing across the face of our star. Again and again this coincidence has been noted — spots on the sun, and magnetic and electrical disturbances on the earth.
‘ We can say, however, that sunspots in themselves do not cause magnetic storms,’ explained Dr. Hulburt. ‘The surface of the sun is like a burning prairie, covered at all times with countless flares and eruptions of flaming gases. Sometimes there is a more violent eruption than usual; it may show in a sunspot or group of spots, or in magnetic disturbances on the earth; usually it shows in both. Thus the sunspot, like the magnetic storm, is not a cause but an effect. Both the spot and the storm are attributable, we believe, to the same cause; and that cause, we feel sure, is within the hot interior of the sun. Our studies show that all the phenomena can be accounted for by outbursts of ultraviolet radiation.’
The solar surface registers an average temperature of about 10,000 degrees Fahrenheit. This is far above the vaporizing point of all known elements. It means that even the heavy metals exist there only as glowing gases, radiating energy in all wave lengths from the heat rays of infra-red to the invisible penetrating rays of ultra-violet. The higher the temperature, the greater the volume and intensity of ultraviolet radiation; therefore, any heating of the solar surface would result in additional outpouring of ultra-violet rays.
What apparently happens is that some explosion within bursts the surface of the sun and exposes an under region where the temperature is 30,000 degrees and more. Through this jagged rent — and sunspots of 50,000 miles diameter have been observed! — pours a blast of ultra-violet radiation. It is an added bombardment which plays havoc with the earth’s upper air. It excites the already excited particles of the Kennelly-Heaviside layer, penetrates the layer into the calmer atmosphere beneath, smashes neutral atoms and molecules by the billions, ionizes them, and enormously increases the population of the electrified layer.
The first effect is to add to the flow of energy that circles continually around the earth. Within an hour the flow may be increased a third or more. It is easy to see that this addition of two or three million horsepower to the energy of the current would intensify the magnetic field of the earth, to the bewitchment of telephone, telegraph, and cable circuits, and of radio waves.
A second effect is the massing of electrified particles above the poles of the earth. The increased bombardment of ultra-violet causes the number of charged particles to accumulate more rapidly than they can be neutralized. As a result two clouds of ions form, one above the North Pole, the other above the South Pole, at a height of about 120 miles. Each of these two concentrations of electrified particles becomes a vast floating bar magnet, with attraction sufficient to account for much of the magnetic and electrical disturbances on the earth.
As the cloud of particles above a polar region becomes more concentrated and more excited with increased ionization, it becomes luminous. Then it is that we see an aurora. The tenuous sky magnet becomes a sign in the heavens, visible (and, some say, audible) token of the mighty sway of the star we are hitched to.
IV
There is another mysterious luminosity in the night sky. One evening last March I saw it just after sunset — a faint band of haze arching up from the western horizon. For centuries it has been called the zodiacal light. But naming a thing does not solve its enigma, and the zodiacal light has long remained an astronomical puzzle. Dr. W. E. Glanville, Episcopal rector of New Market, Maryland, is perhaps the leading observer in the United States of this phenomenon, and has organized an international circle of amateurs to make observations in different parts of the world. He recently published a summary of the various theories that have been advanced to explain the zodiacal light. They range all the way from Cassini’s idea of a meteoric cloud surrounding the sun to Barnard’s less spectacular idea of refracted sunlight.
But Dr. Hulburt secs the zodiacal light as of a piece with all these other effects of particles electrified by solar ultra-violet rays. The finer particles attain speeds which carry them beyond the upper limit of the Kennelly-Heaviside layer, and beyond the great gaseous magnets which quiver above the poles. They finally come to relative rest to form a surrounding cloud about 30,000 miles from the sunlit side of the earth. The pressure of sunlight, continually beating on this ring of cloud, prevents its expansion any farther toward the sun, but that same pressure accelerates its expansion in the opposite direction. As a result, the cloud of fine particles is distended far into space on the night side of the earth, and may attain a distance of 1,000,000 miles, according to Hulburt’s computation.
It is, in effect, a comet-like tail to our planet — a 1,000,000-mile plume of electrified particles that we trail through space as we ride our circuit around the sun at 19 miles a second, and travel toward Vega at 12 miles a second, and partake of the runaway motion of the Milky Way in still another direction at 200 miles a second.
One might think that, with all these motions to accommodate itself to, our plume of electricity might get bent or tangled, but no — the persistent pressure of light both creates and moulds it, so the plume always points away from the sun, always trails the night side of the earth.
Thus, according to Hulburt, the zodiacal light is the last vestige of the earth’s atmosphere, the outermost stuff of our planet. It is too thin and subtile to reflect radio waves or anything else. Perhaps it may be thought of as dust from the radio roof — cosmic dust blown into space by the wind of light that is forever beating against the world.
Far driven, some of this earth stuff escapes, perhaps to be recaptured by a passing planet or meteor, or it may be to travel its own course for millions of years — solitary, an Ishmael in the deserts of space; relic of Earth, symbol of man adrift, tugged at by all the stars, beaten upon by all the radiant forces, but persisting somehow in that ‘freewill of Nature’ which makes the whole universe akin, man and clod.