by Isaac Asimov
In 1673, a French expedition to the north coast of South America (near the Equator) found that, at that location, the pendulum was slowed even at sea level. Newton later took this finding as evidence for the existence of the equatorial bulge, which would lift the camp farther from the earth’s center, and weaken the force of gravity. After the expedition to Peru and Lapland had proved his theory, a member of the Lapland expedition, the French mathematician Alexis Claude Clairault, worked out methods of calculating the oblateness of the earth from pendulum swings. Thus, the geoid, or sea-level shape of the earth, can be determined, and it turns out to vary from the perfect oblate spheroid by less than 300 feet at all points. Nowadays gravitational force is also measured by a gravimeter, a weight suspended from a very sensitive spring. The position of the weight against a scale in the background indicates the force with which it is pulled downward, and hence measures variations in gravity with great delicacy.
Gravity at sea level varies by about 0.6 percent, being least at the Equator, of course. The difference is not noticeable in ordinary life, but it can affect sports records. Achievements at the Olympic Games depend to some extent on the latitude (and altitude) of the city in which they are conducted.
A knowledge of the exact shape of the geoid is essential for accurate map making; and as late as the 1950s, only 7 percent of the earth’s land surface can really be said to have been accurately mapped. The distance between New York and London, for instance, was not known to better than a mile or so, and the locations of some islands in the Pacific were known only within a possible error of several miles. In these days of air travel and (alas!) potential missile aiming, this margin of error is inconvenient. But truly accurate mapping has now been made possible—oddly enough, not by surveys of the earth’s surface but by astronomical measurements of a new kind. The first instrument of these new measurements was the man-made satellite called Vanguard I, launched by the United States on 17 March 1958. Vanguard I revolved around the earth in a period of 2½ hours and, in the first couple of years of its lifetime, had already made more revolutions than the moon had in all the centuries it has been observed with the telescope. By observations of Vanguard I’s position at specific times from specific points of the earth, the distances between these observing points can be calculated precisely. In this way, positions and distances not known to within a matter of miles were, in 1959, determined to within a hundred yards or so. (Another satellite named Transit I-B, launched by the United States on 13 April 1960, was the first of a series specifically intended to extend this into a system for the accurate location of position on the earth’s surface, which could greatly improve and simplify air and sea navigation.)
Like the moon, Vanguard I circles the earth in an ellipse that is not in the earth’s equatorial plane; and also like the moon, the perigee (closest approach) of Vanguard I shifts because of the attraction of the equatorial bulge. Because Vanguard I is far closer to the bulge and far smaller than the moon, it is affected to a greater extent; and because of its many revolutions, the effect of the bulge can be well studied. By 1959, it was certain that the perigee shift of Vanguard I was not the same in the Northern Hemisphere as in the Southern, and thus that the bulge was not quite symmetrical with respect to the Equator. The bulge seemed to be 25 feet higher (that is, 25 feet more distant from the earth’s center) at spots south of the Equator than at spots north of it. Further calculations showed that the South Pole was 50 feet closer to the center of the earth (counting from sea level) than was the North Pole.
Further information, obtained in 1961, based on the orbits of Vanguard I and Vanguard II (the latter having been launched on 17 February 1959) indicates that the sea-level Equator is not a perfect circle. The equatorial diameter is 1,400 feet (nearly a quarter of a mile) longer in some places than in others.
Newspaper stories have described tile earth as “pear-shaped” and the Equator as “egg-shaped.” Actually, these deviations from the perfectly smooth curve are perceptible only to the most refined measurements. No one looking at the earth from space would see anything resembling a pear or an egg, but only what would seem a perfect sphere. Besides, detailed studies of the geoid have shown so many regions of very slight Rattening and very slight humping that, if the earth must be described dramatically, it had better be called “lumpy shaped.”
Eventually satellites, even by methods as direct as taking detailed photographs of the earth’s surface, have made it possible to map the entire world to within an accuracy of a few feet.
Airplanes and ships, which would ordinarily determine their position with reference to stars, could eventually do so by fixing on the signals emitted by navigation satellites—regardless of weather, since microwaves penetrate clouds and fogs. Even submarines below the ocean surface can do so. This can be done with such accuracy that an ocean liner can calculate the difference in position between its bridge and its galley.
WEIGHING THE EARTH
Knowledge of the exact size and shape of the earth makes it possible to calculate its volume, about 260 billion cubic miles. Calculating the earth’s mass, however, is more complex, but Newton’s law of gravitation gives us something to begin with. According to Newton, the gravitational force (f) between any two objects in the universe can be expressed as follows:
f = gm1m2
d²
where m1 and m2 are the masses of the two bodies concerned, and d is the distance between them, center to center. As for g, that represents the gravitational constant.
What the value of the constant was, Newton could not say. If we can learn the values of the other factors in the equation, however, we can find g; for by transposing the terms, we get:
g = fd²
m1m2
To find the value of g, therefore, all we need to do is to measure the gravitational force between two bodies of known mass at the separation of a known distance. The trouble is that gravitational force is the weakest force we know, and the gravitational attraction between two masses of any ordinary size that we can handle is almost impossible to measure.
Nevertheless, in 1798, the English physicist Henry Cavendish, a wealthy, neurotic genius who lived and died in almost complete seclusion but performed some of the most astute experiments in the history of science, managed to make the measurement. Cavendish attached a ball of known mass to each end of a long rod and suspended this dumbbell-like contraption on a fine thread. Then he placed a larger ball, also of known mass, close to each ball on the rod—on opposite sides, so that gravitational attraction between the fixed large balls and the suspended small balls would cause the horizontally hung dumbbell to turn, thus twisting the thread (figure 4.1). The dumbbell did indeed turn slightly. Cavendish now measured how much force was needed to produce this amount of twist of the thread. This told him the value of f. He also knew m1 and m2, the masses of the balls, and d, the distance between the attracted balls. So he was able to compute the value of g. Once he had that, he could calculate the mass of the earth, because the earth’s gravitational pull (f) on any given body can be measured. Thus Cavendish “weighed” the earth for the first time.
Figure 4.1. Henry Cavendish’s apparatus for measuring gravity. The two small balls are attracted by the larger ones, causing the thread on which they are suspended to twist. The mirror shows the amount of this slight twist by the deflection of reflected light on the scale.
The measurements have since been greatly refined. In 1928, the American physicist Paul R. Heyl at the United States Bureau of Standards determined the value of g to be 0.00000006673 dyne centimeter squared per gram squared—a number since refined to 0.000000066726. You need not be concerned about those units, but note the smallness of the figure. It is a measure of the weakness of gravitational force. Two 1-pound weights placed 1 foot apart attract each other with a force of only one-half of one billionth of an ounce.
The fact that the earth itself attracts such a weight with the force of 1 pound even at a distance of 3,960 miles from its center emp
hasizes how massive the earth is. In fact, the mass of the earth turns out to be 6,585,000,000,000,000,000,000 tons or, in metric units, 5,976,000,000,000,000,000,000,000 kilograms.
From the mass and volume of the earth, its average density is easily calculated. In metric units, the answer comes out to 5.518 grams per cubic centimeter (5.518 times the density of water). The density of the earth’s surface rocks averages only about 2.8 grams per cubic centimeter, so the density of the interior must be much greater. Does it increase smoothly all the way down to the center? The first proof that it does not—that the earth is made up of a series of different layers—came from the study of earthquakes.
Earth’s Layers
EARTHQUAKES
There are not many natural disasters that can, in five minutes, kill hundreds of thousands of people. Of these, by far the most common is the earthquake.
The earth suffers a million quakes a year, including at least 100 serious ones and 10 disastrous ones. The most murderous quake is supposed to have taken place in the northern province of Shensi in China in 1556, when 830,000 people were killed. Other quakes nearly as bad have also taken place in the Far East. On 30 December 1703, an earthquake killed 200,000 people in Tokyo, Japan; and on 11 October 1737, one killed 300,000 people in Calcutta, India.
In those days, though, when science was developing in western Europe, little attention was paid to events that took place on the other side of the world. But then came a disaster much closer to home.
On 1 November 1755, a great earthquake, possibly the most violent of modern times, struck the city of Lisbon, demolishing every house in the lower part of the city. Then what is called a tidal wave swept in from the ocean. Two more shocks followed, and fires broke out. Sixty thousand people were killed, and the city was left a scene of devastation.
The shock was felt over an area of one and a half million square miles, doing substantial damage in Morocco as well as in Portugal. Because it was All sours Day, people were in church, and it is said that all over southern Europe those in the cathedrals saw the chandeliers dance and sway.
The Lisbon disaster made a great impression on the scholars of the day. It was an optimistic time when many thinkers felt that the new science of Galilco and Newton would give human beings the means of making the earth a paradise. This blow showed that there were still giant, unpredictable, and apparently malicious forces beyond human control. The earthquake inspired Voltaire, the great literary figure of the time, to write his famous pessimistic satire Candide, with its ironical refrain that all is for the best in this best (If all possible worlds.
We are accustomed to thinking of dry land as shaking with the effect of an earthquake, but the earth beneath the ocean floor may be set to quivering too, with even more devastating effects. The vibration sets up long, gentle swells in the ocean which, on reaching the shallow shelves in the neighborhood of land—particularly when driven into the narrowing confines of a harbor—pile up into towers of water, sometimes 50 to 100 feet high. If the waves hit with no warning, thousands of people are drowned. The popular name of “tidal wave” for such earthquake-generated waves is a misnomer. They may resemble monstrous tides, but they have entirely different causes. Nowadays, they are referred to by the Japanese name tsunami (“harbor wave”). Japan’s coastline is particularly vulnerable to such waves, so this nomenclature is justified.
After the Lisbon disaster, to which a tsunami had added its share of destruction, scientists began turning their thoughts earnestly to the possible causes of earthquakes. The best theory of the ancient Greeks (aside from the thought that they were caused by the angry writhing of giants imprisoned underground) had been Aristotle’s suggestion that they was caused by masses of air, imprisoned underground and trying to escape. Modern scientists, however, suspected that earthquakes might be the effect of earth’s internal heat on stresses within the solid rock itself.
The English geologist John Michell (who had studied the forces involved in torsion, or twisting, later used by Cavendish to measure the mass of the earth) suggested in 1760 that earthquakes are waves set up by the shifting of masses of rock miles below the surface, and it was he who first suggested that tsunamis are the result of undersea earthquakes.
To study earthquakes properly, an instrument for detecting and measuring these waves had to be developed, and this did not come to pass until one hundred years after the Lisbon quake. In 1855, the Italian physicist Luigi Palmieri devised the first seismograph (from Greek words meaning “earthquake writing”).
Palmieri’s invention consisted of a horizontal tube turned up at each end and partly filled with mercury. Whenever the ground shook, the mercury moved from side to side. It responded to an earthquake, of course, but also to any other vibration, such as that of a cart rumbling along a road nearby.
A much better device, and the ancestor of all those used since, was constructed in 1880 by an English engineer, John Milne. Five years before, he had gone to Tokyo to teach geology and mining and there had ample opportunity to study earthquakes, which are common in Japan. His seismograph was the result.
In its simplest form, Milne’s seismograph consists of a massive block suspended by a comparatively weak spring from a support firmly fixed in bedrock. When the earth moves, the suspended block remains still, because of its inertia. However, the spring attached to the bedrock stretches or contracts a little with the earth’s motion. This motion is recorded on a slowly rotating drum by means of a pen attached to the stationary block, writing on smoked paper. Actually, two blocks are used: one oriented to record the earthquake waves traveling north and south; the other, east and west. Ordinary vibrations, not originating in bedrock, do not affect the seismograph. Nowadays, the most delicate seismographs, such as the one at Fordham University, use a ray of light in place of a pen, to avoid the frictional drag of the pen on the paper. This ray shines on sensitized paper, making tracings that are developed as a photograph.
Milne was instrumental in setting up stations for the study of earthquakes and related phenomena in various parts of the world, particularly in Japan. By 1900, thirteen seismograph stations were in existence, and today there are over 500, spread over every continent including Antarctica. Within ten years after the establishment of the first of these, the correctness of Michell’s suggestion that earthquakes are caused by waves propagated through the body of the Earth was clear.
This new knowledge of earthquakes did not mean that they occurred less frequently, or that they were less deadly when they did occur. The 1970s, in fact, were rich in severe earthquakes.
On 27 July 1976, an earthquake in China destroyed a city south of Peking and killed about 650,000 people. This was the worst disaster of the sort since the one in Shensi four centuries before. There were other bad earthquakes in Guatemala, Mexico, Italy, the Philippines, Rumania, and Turkey.
These earthquakes do not mean that our planet is growing less stable. Modern methods of communication make it certain that we hear of all earthquakes everywhere—often with instant eyewitness scenes, thanks to television—where in earlier times (even a few decades ago) distant catastrophes would have gone unreported and unnoticed. What’s more, earthquakes are more likely to be catastrophic now than in earlier times (even a century ago), since there are many more people on Earth, crowded much more intensively into cities, and because man-made structures, vulnerable to earthquakes, are much more numerous and expensive.
All the more reason to work out methods for predicting earthquakes before they occur. Seismologists are seeking for significant changes. The ground might hump up in places. Rocks might pull apart or squeeze together, absorbing water or squeezing it out, so that rises and falls in well water might be significant. There might be changes in the natural magnetism of rocks or in electrical conductivity. Animals, aware of tiny vibrations or alterations in the environment, which human beings are too busy to notice, may begin to react in a nervous manner.
The Chinese, in particular, have take
n to collecting all reports of anything unusual, even flaking paint, and report that an earthquake in northeastern China on 4 February 1975 was predicted. People therefore left their homes for the open fields outside the city, and thousands of lives were saved. However, the more serious earthquake of 1976 was not predicted.
There is also the point that until predictions are more certain than they are now, warnings may do more harm than good. A false alarm could disrupt life and the economy and do more harm than a mild quake could. Furthermore, after one or two false alarms, a correct prediction might be ignored.
The damage an earthquake can do is not surprising. The largest earthquakes are estimated to release a total energy equal to 100,000 ordinary atomic bombs or, if you prefer, 100 large hydrogen bombs. It is only because earthquakes’ energies are dissipated over a large area that they are not even more destructive than they are. They can make the earth vibrate as though it were a gigantic tuning fork. The Chilean earthquake of 1960 caused our planet to vibrate at a frequency of just under once an hour (20 octaves below middle C and quite inaudible).
Earthquake intensity is measured on a scale from 0 up through 9, where each number represents an energy release about 31 times that of the number below. (No quake of intensity greater than 9 has ever been recorded, but the Good Friday quake in Alaska in 1964 recorded an intensity of 8.5.) This is called the Richter scale because it was introduced in 1935 by the American seismologist Charles Francis Richter.
One favorable aspect of earthquakes is that not all the earth’s surface is equally exposed to their dangers (though this is cold comfort to those who live in regions that are so exposed).
About 80 percent of earthquake energy is released in the areas bordering the vast Pacific Ocean. Another 15 percent is released in an east-west band sweeping across the Mediterranean. These earthquake zones (see figure 4.2) are closely associated with volcanic areas—one reason the effect of internal heat was associated with earthquakes.
