Popular Science Monthly/Volume 31/July 1887/Earthquakes
| EARTHQUAKES. |
By Professor G. H. DARWIN, F. R. S.
THE earthquake-shocks which have recently occurred in America and Greece, and the great volcanic eruption in New Zealand, have served to keep the subject vividly before us during many months past, and have perhaps created in some alarmist minds an ungrounded expectation that the earth is about to enter on a new period of Plutonic activity. It is natural, then, to ask at the present time what is an earthquake, and what are its causes. Notwithstanding the necessary incompleteness in the answers which can be given to these questions, yet a good deal more is known than appears to be the common property of newspaper writers. The object, then, of the present article is to give a rough sketch of the present state of knowledge in this complicated subject.
Although history abounds with more or less complete accounts of earthquakes, it is remarkable that hardly ten years have yet elapsed since an accurate record was first obtained of what actually occurs during an earthquake. The combination of circumstances is curious, by which a knot of Scotch students, working in Japan, has secured so considerable an advance in seismology. The incessant but usually non-destructive earthquakes by which Japan is visited, the strange Japanese renaissance, and the importation of foreign professors, thoroughly trained at the Scotch universities in an accurate perception of mechanical principles, are the three factors which have co-operated to bring about this result.
The Scoto-Japanese professors, of whom the most eminent are Ewing, Gray, and Milne, have studied their subject with admirable persistence, and have by their ingenuity placed seismologists in possession of instruments by which the motion of the ground during an earthquake is recorded on an accurate scale of time. Such instruments are called seismographs, or recording seismometers. During an earthquake the ground and all that is fixed to it move together, and at first sight it seems impossible to get anything to stay still during the vibration. An exact description of a scientific instrument would be out of place here, but a general notion of these seismographs may be easily grasped.
The horizontal pendulum of Zöllner, and a suggestion of Chaplin (also a professor in Japan), are the sources from which "the horizontal pendulum seismograph" of Ewing originated. The principles according to which it is constructed may be explained as follows: If we consider an open door which can swing on its hinges, and imagine that a sudden horizontal movement is given to the door-post, at right angles to the position in which the door is hanging, then it is clear that the outer edge of the door will begin to move with a sort of recoil in the direction opposite to that of the movement imparted to the door-post. Since the door-post moves in one direction, while the edge of the door recoils, somewhere in the door there is a vertical line which remains still. The exact position of this line depends on the proportion which the amount of the recoil of the outer edge bears to the direct motion of the door-post. Now, if the sudden movement is imparted to the door-post by means of the floor to which it is attached, it is clear that a pencil attached to the door at this vertical line will write on the floor the displacement of the door-post, notwithstanding that the floor has moved. If next we suppose that there are two such doors hanging at right angles to one another from the same door-post, and that a sudden horizontal movement in any direction is given to the floor, each pencil will write on the floor that part of the movement which was at right angles to its door. Lastly, if the floor or surface on which the record is written is kept moving uniformly by clock-work, we obtain also a register in time as well as space.
But in an earthquake the surface of the earth undergoes also a vertical movement which has to be recorded. The principle by which an instrument may be constructed to attain this end is as follows: If a weight hangs by a long, elastic cord, so that when set dancing up and down it oscillates very slowly, then, when a sudden jerk is given to the point of support, the weight will for the moment stand almost stationary, and a pencil attached to it may write its record on a surface fastened to the part jerked. This idea has been utilized in the construction of a vertical seismograph, but various important modifications have been introduced for the purpose of annulling the spontaneous dance of the weight after the shock has occurred.[1]
It will undoubtedly serve to give an impulse to this science that henceforth the intending observer need not waste time in devising and constructing instruments, but can purchase the complete equipment of a seismological observatory, as recommended by Ewing, and may begin work at once.
Many other instruments besides these have been used for the observation of earthquakes, and among the best are those of Bertelli, Rossi, and Palmieri. An instrument which tells only that there has been a shock, without giving a record of the nature of the movement, is called a seismoscope. Some of the Italian instruments are seismoscopes, which, however, give an approximate idea of the severity and direction of the vibration, and others claim to be accurate seismographs or seismometers. But I do not think that any of them can compete with the instruments described in outline above.
And what do recording instruments tell us of the actual occurrences during an earthquake?
"They show," writes Ewing,[2] "that, as observed at a station on the surface of the earth, an earthquake consists of a very large number of successive vibrations—in some cases as many as three hundred have been distinctly registered. They are irregular both in period and amplitude, and the amplitude does not exceed a few millimetres "(a millimetre is one twenty-fifth of an inch)," even when the earthquake is of sufficient severity to throw down chimneys and crack walls, while in many instances the greatest motion is no more than the fraction of a millimetre. The periods of the principal motions are usually from half a second to a second, but ... the early part of the disturbance often contains vibrations of much greater frequency. The earthquake generally begins and always ends very gradually, and it is a noteworthy fact that there is in general no one motion standing out from the rest as greatly larger than those which precede and follow it. The direction of motion varies irregularly during the disturbance—so much so, that in a protracted shock the horizontal movements at a single station occur in all possible azimuths" (that is to say, to all points of the compass). "The duration, that is to say the time, during which the shaking lasts at any one point is rarely less than one minute, often two or three, and in one case in the writer's experience was as much as twelve minutes."
The horizontal path pursued, in an actual earthquake at Tokio, on March 8, 1881, by the part of the recording instrument which was fixed 
to the ground, is shown in the annexed figure.[3] It is magnified six fold, and the time occupied from the beginning to the end of this part of the vibration was three seconds. This earthquake, although alarming, did no damage except to crack a few walls.
It is obvious that when the motion is so complicated, the impressions of people present go for little as compared with an automatic record. Observers often differ widely among themselves as to what was the direction of the prevailing oscillation, and the magnitude of the displacement of the ground is generally much exaggerated. It is true that in some of the great historic earthquakes the displacements are supposed to have been considerable; for example, according to Mallet, in the Neapolitan shock of 1857 it amounted to a foot, and Abella assigns six feet as the amplitude in the Manila earthquake of 1881. But, without contesting the accuracy of these estimates, it is safe to say that such displacements are very rare, for, as proved by automatic seismographs, when the motion is as much as a quarter of an inch, brick and stone chimneys are generally shattered.
Every railway-traveler knows that it is not the steady speed, but the starting and stopping, which jars him; that is to say, it is change of velocity by which he is shaken. The misconception of an observer in an earthquake arises from the fact that the sensation of being tossed about comes from the change of velocity to which he is subjected, rather than from the extent of his displacement. Now, the greatest change per second of velocity may be considerable in a vibration, while the amplitude is small.
The force of gravity is the most familiar example of a change per second of velocity, for in each second the velocity of a falling body is augmented by a velocity of thirty-two feet a second. Ewing appears to have been the first to think of comparing the greatest change per second of velocity in an earthquake with gravity. Thus at Tokio, on March 11, 1882, walls were cracked and chimneys knocked over, and in this shock the greatest change per second of velocity may be expressed by the phrase one thirty-fifth of gravity; in other words, the greatest change per second of velocity was 3235 of a foot per second. This conclusion enables us also to illustrate the mechanical consequences of the shock in another way; for if a wall thirty-five feet high leans over, so that the top brick projects a foot beyond the bottom brick, the forces tending to upset the wall are the same as those which occurred in this earthquake.
No great shock has ever yet been recorded by automatic instruments, and it is not unlikely that in these great disasters the instruments would be thrown out of gear, and no record would be obtained. Thus, earthquakes which only work a moderate amount of destruction are the most favorable for scientific operation.
Since the oscillations at any one spot are usually in all sorts of directions, it is impossible, from observation at a single place, to form a sound opinion as to position of the origin of the disturbance. Much information useful for the study of vibrations and of the laws of their decrease with increasing distance, has resulted from a laborious series of experiments made by Milne at Tokio. Artificial earthquakes were produced by the explosion of gun-cotton in holes in the ground, and by the fall of heavy weights, and the records at various distances from the origin were obtained.
From theoretical considerations, confirmed by these experiments, it is established that earthquake-waves consist of oscillations of two kinds, namely, waves or vibrations of compression, and of distortion. In the first kind the motion of each particle of the ground is to and fro in the direction in which the wave is traveling; and in the second kind the excursions are at right angles to the direction of wave-propagation. As the former vibrations travel more rapidly than the latter, all the compressional waves may have passed a given station before the arrival of the distortional waves, and thus the shock may be apparently duplicated. Or, nearer to the origin, the two series will overlap, and a complex movement ensues, such as that exhibited in the figure above. The phenomena are further complicated by frequent reflections and refractions, as the wave passes from one geological stratum to another. The rate at which these waves travel depends on the nature of the rock through which the movement passes; velocities ranging from a mile per second to five miles per second are usual.
The destructive effects of earthquakes on buildings are notorious, and it is unnecessary to describe them here. By an examination of ruined buildings a competent observer is often able to obtain a good deal of information as to the nature of the shock. Thus Mallet visited the towns destroyed by the Neapolitan earthquake of 1857, and, by a very careful consideration of the fractures in walls and other damage, was able to draw a number of interesting conclusions as to the directions and amplitudes of the principal vibrations and as to the site of the center of disturbance.
Architects should be able, by an adherence to sound mechanical principles, to construct buildings which should stand against all but the severest shocks, and much has already been done in this way. Where a choice for the site of an intended building is possible, the most important consideration is that it should be where there has been the greatest immunity from vibration on previous occasions, for, even within a very small area, different spots are very differently affected. In most regions there is only a single important center whence earthquakes originate, and the safe places are situated in what may be called earthquake-shadow for the prevalent vibrations. For just as a high wall, a hill, or a railway-cutting often completely cuts off sounds by forming a sound-shadow, so a ravine or some arrangement of the geological formation may afford earthquake-shadow for particular places.
It is not in general possible to pick out the favorable sites by mere inspection, for the distribution of vibration is often apparently capricious. Thus Milne tells us of a princely mansion at Tokio "which has so great a reputation for the severity of the shakings it receives, that its marketable value has been considerably depreciated, and it is now untenanted."[4]
In a town which is frequently shaken there is no need to wait long to carry out a rough survey with seismographs, and thus to obtain an idea of the relative shakiness of the various parts. If such a survey is impossible, it is best to avoid as the site for building a loose soil, such as gravel, resting on harder strata, and the edge of a scarp or bluff, or the foot of similar eminences.[5]
The same capriciousness of distribution, which is observable on a small scale, is found to hold on a large scale when we consider the distribution of earthquakes throughout a whole country. Regions subject to earthquakes, or seismic areas, appear to have fairly definite boundaries, which remain constant for long periods. For example, in Japan, earthquakes are rarely felt on the western side of the central range of mountains.
The search for the actual point whence the earthquake originated is one of the most interesting branches of the science. In order to trace the earthquakes in a country to their origin, the places of observation should not be chosen where there is comparative immunity from shaking. Thus a seismic survey is necessary, and the limits of the seismic areas will be discovered in the course of it. Milne commenced his survey of Japan by sending to the local government offices in the important towns, distant from thirty to a hundred miles from Tokio, packets of post-cards, one of which was to be returned to him at the end of each week with a record of the shocks which had been felt. "The barricade of post-cards was then extended farther northward, with the result of surrounding the origin of certain shocks among the mountains, while others were traced to the sea-shore. By systematically pursuing earthquakes, it was seen that many shocks had their origin beneath the sea, ... but it was seldom that they crossed through the mountains forming the backbone of the island."[6] When the country had been thus mapped out, it was possible to choose the most advantageous sites for the observatories.
It would carry us too far into technical matters to describe the method of searching into the bowels of the earth for the actual point of disturbance. It must here suffice to say that if a shock be accurately-timed at various places, and if the approximately circular ring where it was most severe be determined, it is possible to find with fair accuracy the spot or spots under which it originated and the depth of the earthquake-center. Even without accurate time-observations, Mallet was able to show that the Neapolitan shock originated between three and seven miles below the surface. The Yokohama earthquake of 1880 appears to have had its center at a depth of from one and a half to five miles. Notwithstanding that one earthquake has been estimated as originating at a depth of fifty miles, it is probable that in all cases the center of shock is only a few miles below the surface.
The vagueness as to the position which has been assigned for the center of disturbance in the case of particular earthquakes probably depends less on the difficulty of tracing back the vibrations to their origin, than on the fact that the shocks do not usually originate in a single point, but rather along a line of a mile or two in length.
As to the way in which seismic activity is distributed geographically over the earth's surface, certain broad conclusions have been fairly well ascertained. If a map be shaded, so as to represent the frequency of earthquakes, we see that the shading has a tendency to fall into bands or ribbons, which generally follow the steeply sloping shores of continents and islands; and it is probable that the actual origins of the shocks are generally situated under the sea not far from the coast.
It is a further interesting peculiarity that the most important bands fall end to end, so that they may be regarded as a single ribbon embracing nearly half the earth. It may be suspected that this ribbon really meets itself and forms a closed curve, but this can not be proved as yet. We may begin to trace its course at Cape Horn, whence it follows the Andes along the whole western coast of South America. At the north of that continent it becomes somewhat broader, but its course is clearly marked along the line of the West Indies from Trinidad to Cuba. Hence it passes to the mainland in Mexico, and then runs along the whole western coast of North America. We then trace the line
- through, the Aleutian Islands to Kamchatka, and thence southward
through the Japanese Islands, the Philippines, and the Moluccas, to Sumatra and Java. Another branch seems to run from Sumatra, through New Guinea, to New Zealand, and the closed curve may perhaps be completed through the Antarctic regions, which are known to be volcanic. Returning to the first branch which we traced as far as Java, to the westward the seismic areas become more patchy and less linear. It may, however, perhaps be maintained that the ribbon runs on through India, Persia, Syria, the Eastern Mediterranean, Greece, and Italy.
This grouping of seismic areas into a ribbon does not comprise all the regions of earthquakes, but it must rather be taken as meaning that there is one great principal line of cracking of the earth's surface. In speaking here of earthquakes, those sensible shocks are meant which are sufficiently severe to damage buildings, for, as will be explained below, there is reason to believe that the whole earth is in a continual state of tremor.
Seismic areas are not absolutely constant in their limits, and cases are known where regions previously quiescent have become disturbed. It seems likely that the recent disastrous earthquake at Charleston belongs to the West Indian system of seismic activity, but there is no reason to suspect a permanent extension of the West Indian area so as to embrace the Southern States. On the contrary, it is far more probable that this disastrous shock will remain a unique occurrence. The previous experience of great earthquakes, such as that of Lisbon in the middle of the last century, shows, however, that the inhabitants of Charleston must for the next year or two expect the recurrence of slight shocks, and that the subterranean forces will then lull themselves to sleep again.
With regard to the distribution of earthquakes in time there is no evidence of either decrease or increase within historical periods, and although physical considerations would lead us to suppose that they were more frequent in early geological times, geology at least can furnish no proof that this has been the case.[7]
A great deal has been written on the causes of earthquakes, and many of the suggested theories seem fanciful in the highest degree. It is clear, however, that the primary cause resides in the upper layers of the earth, and that the motive power is either directly or indirectly the internal heat of the earth. The high temperature of the rocks, in those little scratches in the earth's surface which we call mines, proves the existence of abundant energy for the production of any amount of disturbance of the upper layers. It only remains to consider how that energy can be brought to bear. One way is by the slow shrinking of the earth, consequent on its slow cooling. Now, the heterogeneity of the upper layers makes it impossible that the shrinkage shall occur with perfect uniformity all round. Thus, one part of the surface will go down before another, and as this must usually occur by a cracking and sudden motion, the result will be an earthquake.
The seismic ribbons of which we have spoken are probably lines of weakness along which cracking habitually takes place. Along these lines there are enormous dislocations of the geological strata, and earthquakes are known to follow lines of faulting. The geologically recent elevation of the Pacific coast of South America is obviously, from this point of view, connected with the abundance of volcanoes and the frequency of earthquakes along the chain of the Andes.
One would think that the continual ejection of lava and ashes from an active volcano must leave a hollow under the mountain, and that some day the cavern would suddenly collapse. It has, however, been observed that volcanic eruptions and severe earthquakes are to some extent alternatives, so that it seems as though the volcanic vent served as a safety-valve for the liberation of the dangerous matter. But the theory of the collapsing cavern must not be wholly rejected, for some disastrous earthquakes affecting only very restricted areas, such as that of Casamicciola in Ischia, are hardly otherwise explicable. In this case Palmieri has attributed the formation of the cavern to evisceration under the town produced by hot mineral springs.
In the theories of which we have just spoken, the internal heat of the earth acts indirectly, by giving to gravitation an opportunity of coming into play. But as in volcanic eruptions enormous quantities of steam are usually emitted, it is probable that the pressure of steam is the force by which the lava and ashes are vomited forth, and that the steam is generated when water has got among hot internal rocks. From this point of view we can understand that an eruption will serve as a protection against earthquakes, and that the centers of disturbance will usually be submarine.
It may on the whole be safely concluded that a diversity of causes are operative, and that some earthquakes are due to one and others to other causes.
It would, however, be certainly wrong to look only to the interior of the earth for the causation of earthquakes, since the statistics of earthquakes clearly point to connections with processes external to the solid earth.
The laborious inquiries of M. Perrey show that there are more earthquakes at the time of full and change of moon than at other times, more when the moon is nearest to the earth and more when she is on the meridian than at the times and seasons when she is not in those positions relatively to the earth. The excess of earthquakes at these times is, however, not great, and an independent investigation of the Japanese earthquakes does not confirm Perrey's results. It is well, therefore, still to hold opinion in suspense on this point. If, however, Perrey's result should be confirmed, we must attribute it to the action of those forces which produce tides in the ocean, and therefore at the same time cause a state of stress in the solid earth.
Then again it is found that earthquakes are indubitably more apt to occur when there is a rapid variation of the pressure of the air, indicated by a rise or fall of the barometer, than in times of barometric quiescence. It is certain that earthquakes in both hemispheres are more frequent in the winter than in the summer; this is probably connected with the greater frequency of sudden rises and falls of the barometer at that season. It may, however, be urged against this view that volcanic eruptions are somewhat more frequent in the summer. But whatever be the action of these external processes with regard to earthquakes, it is certain that the connection between the two is merely that of the trigger to the gun. The internal energy stands waiting for its opportunity, and the attraction of the moon or the variation in atmospheric pressure pulls the trigger. Thus the predictions of disaster which have frequently been made for particular dates must be regarded as futile.
It has long been known that an earthquake is preluded by slight tremors leading by a gradual crescendo to the destructive shocks. But within the last fifteen years it has been discovered that the earth's surface is being continually shaken by tremors, so minute as to remain unsuspected without the intervention of the most delicate instruments. In every country where the experiment has been tried, these tremors have been detected, and not merely at certain periods, but so incessantly that there is never a second of perfect rest. The earth may fairly be said to tremble like a jelly. The pioneer in this curious discovery was Father Bertelli. His experiments relate only to Italy, but that which has been found true also of England, France, Egypt, Japan, Brazil, and a solitary island in the South Pacific Ocean, probably holds good generally, and we may feel sure that earth-tremors or "microseisms" are not confined to countries habitually visited by the grosser sort of earthquakes.
Almost all our systematic knowledge of microseisms comes from Italy, for a co-operation has been arranged there between many observers with ingenious instruments at their disposal. Besides Bertelli, the most eminent of the observers is Cavaliere Michele de Rossi, who has established at Rome a "Geodynamical Observatory," and has initiated as an organ of publication the "Bulletino del Vulcanismo Italiano," in whose pages are to be found contributions from Malvasia, Monte, Cecchi, Palmieri, Egidi, Galli, and many others. The literature which has already accumulated on the subject is extensive, and it will only be possible generally to indicate its scope.
The Italians have, of course, occupied themselves largely with earthquakes, but in that field their results do not present a great deal that is novel. The instruments in use for the observation of microseisms are scarcely to be classed as perfect seismographs or seismometers, and the minuteness of the movements to be observed no doubt entails especial difficulties. The "normal tromometer" of Bertelli and Rossi is a simple pendulum, about six feet long, with an arrangement for observing the dance of the pendulum-bob with a microscope. With this and other instruments it has been established that the soil of Italy trembles incessantly. The agitation of the pendulum is usually relatively considerable for about ten days at a time; toward the middle of the period it increases in intensity, when there generally ensues an earthquake which can be perceived without instruments; the agitation then subsides. This has been called by Rossi a seismic period or seismic storm. After such a storm there ensues a period of a few days of relative quiescence.
The vibration of the pendulum in these storms is in general parallel to neighboring valleys or chains of mountains, and its intensity seems to be independent of wind, rain, and temperature. Care is of course taken not to mistake the tremors due to carts and carriages for microseismic agitation, and it has been found easy to effect this separation. The positions of the sun and moon exercise some influence on these tremors, but the most important concomitance which has been established is that they are especially apt to be intense when the barometer is low.
Microseismic storms are not strictly simultaneous at different places in Italy; but if a curve be constructed to represent the average intensity of agitation during each month, it is found on comparison of the curves for a year—for, say, Rome, Florence, and Leghorn—that there is a very close agreement between them.
Rossi has also made some interesting experiments with the microphone on microseisms. In this instrument one electrical conductor is arranged to rest on another at a single point—say, a nail resting on its point on a shilling. One copper wire is attached to the nail and another to the shilling, and an electric current, with an ordinary telephone receiver in the circuit, is then passed through the system. As long as the microphone is still, nothing is heard; but on the occurrence of the very slightest tremor, a noise is audible in the telephone. The instrument can be made so sensitive that a fly may be heard to walk near the microphone with a loud tramp, and a touch with a hair to the nail or to the shilling would sound like the grating of a harsh saw.
Rossi placed his microphone on the ground in a cavern sixty feet below the surface, on a lonely part of Rocca di Papa, an extinct volcano not far from Rome, while he listened with his telephone at the surface of the earth. He then heard the most extraordinary noises, which, as he says, revealed "natural telluric phenomena."
The sounds he describes as "roarings, explosions occurring isolated or in volleys, and metallic or bell-like sounds." They all occurred mixed together, and rose and fell in intensity at irregular intervals. He found it impossible by any artificial disturbance to a microphone to produce the greater number of these noises. The microphone is especially sensitive to vertical movements of the soil, whereas the tromometer fails to reveal them. Nevertheless, there was more or less concordance between the agitations of the two instruments. In order, then, to determine the noises corresponding to various kinds of oscillation, he transported his microphone to Palmieri's Vesuvian observatory, where mild earthquakes are almost incessant; here he discovered that each class of shock had its characteristic noise. The vertical shocks gave the volleys of musketry and the undulatory shocks the roarings. By a survey with his microphone he concluded that the mountain is divided by lines of approximate stillness into regions where the agitation is great. If a metal plate dusted over with sand is set into vibration by a violin-bow rubbing on its edge, all the sand congregates into lines which mark out a pattern on the plate: these lines are nodes, or lines of stillness. It appears, then, that, when Vesuvius trembles with earthquake-shocks, its method of vibration is such that there are nodes of stillness.
At the Solfatara of Pozzuoli the sounds were extraordinarily loud; and the prevailing noise could be imitated by placing the microphone on the lid of a boiling kettle. Similar experiments have since been made by Milne in Japan with similar results.
Some years ago my brother Horace and I made some experiments at Cambridge with a pendulum, so arranged as to betray the minutest displacements. It was then but few years since Bertelli and Rossi had begun to observe; we had read no account of their work, and earth-tremors were quite unsuspected by us. Indeed, the object of our experiment, the measurement of the moon's attraction on a plummet, was altogether frustrated by these disturbances. The pendulum was successfully shielded from the shaking caused by traffic in the town, so that there was no perceptible difference in its behavior in the middle of the night on Sunday, and in the day-time during the week. We were then much surprised to find that the dance of the pendulum (for it was not a regular oscillation) was absolutely incessant. The agitation was more marked at some times than at others; the relatively large swinging, though absolutely very small, would continue for many days together, and this would be succeeded by a few days of comparative calm. In fact, we saw the seismic storms and calm of the Italians.[8] As the instrument was designed for another purpose, and was not quite appropriate for microseismic observation, we did not continue to note it after a month or two. But the substantial identity of the microseisms of England and Italy seems fairly well established.
The cause of these interesting vibrations are as yet but little understood, and it may be hoped that the subject will receive further attention. It seems probable that they are in part true microscopic earthquakes, produced by the seismic forces in the neighborhood. But they are also doubtless due to the reverberation of very distant shocks. It is probable that there is not a minute of time without its earthquake somewhere, and the vibrations may often be transmitted to very great distances. In only a very few cases has it hitherto been possible to identify a tremor with a distant shock, and even then the identification is necessarily rather doubtful. One of the best authenticated of these cases was when M. Nyrèn, an astronomer at St. Petersburg, noticed on May 10 (April 28), 1877, a very abnormal agitation of the levels of his telescope, an hour and fourteen minutes after there had been a very severe shock at Iquique, in Peru.
Astronomers are much troubled by slight changes in the level of the piers of their instruments, and they meet this inconvenience by continually reading their levels and correcting their results accordingly. Of course, they also take average results. These troublesome changes are probably earth-tremors, with so slow a motion to and fro that the term tremor becomes inappropriate. This kind of change has been called a displacement of the vertical, since a plummet moves relatively to the ground. Thus, we found at Cambridge that as the pendulum danced it slowly drifted in one direction or the other. There was a fairly regular daily oscillation, but the pendulum would sometimes reverse its expected course for a few minutes, or for an hour. During the whole time that we were observing, the mean position of the pendulum for the day slowly shifted in one direction; but even after a voyage of six weeks the total change was still excessively small. How far this was a purely local effect and how far general we had no means of determining.
This is a subject which M. d'Abbadie, of the French Institute, has made especially his own. Notwithstanding his systematic observations, carried on during many years in an observatory near the Bay of Biscay, on the French side of the Spanish frontier, hardly anything has been made out as to the laws governing displacements of the vertical. He has, however, been able to show that there is a tendency for deflection of the vertical toward the sea at high tide, but this deflection is frequently masked by other simultaneous changes of unexplained origin.
This result, and the connection between barometric variations and earthquakes and tremors, should make us reflect on the forces which are brought into play by the rise and fall of the tide and of atmospheric pressure. Our very familiarity with these changes may easily blind us to the greatness of the forces which are so produced. The sea rests on the ground, and when the tide is high there is a greater weight to be supported than when it is low. A cubic foot of water weighs 62 pounds; thus if high-tide be only ten feet higher than low-tide, every square foot of the sea-bottom supports 620 pounds more at high than at low water; and 620 pounds to the square foot is nearly 8,000,000 tons to the square mile. Again, the barometer ranges through fully two inches, and a pool of mercury two inches deep and a square foot in area weighs 145 pounds; hence, when the barometer is very high, every square foot of the earth-surface supports about 140 pounds more than if it is low; and 140 pounds to the square foot is 1,800,000 tons to the square mile.
Now, rocks are not absolutely rigid against flexure, certainly less so than most of the metals, and these enormous weights have to be supported by the rocks. Taking a probable estimate for the elasticity of rocks, I have made some calculations as to the amount of effect that we may expect from this shifting of weights, and I find that it is likely that we are at least three or four inches nearer the earth's center when the barometer is very high than when it is very low.[9]
It may be that the incessant straining and unstraining of the earth's surface is partly the cause of earth-tremors, and we can at least understand that these strains may well play the part of the trigger for precipitating the explosion of the internal seismic forces. The calculations also show that near the sea-coast the soil must be tilted toward the sea at high-water, and that the angle of tilting may be such as could be detected by a delicate instrument like that of M. d'Abbadie.
This breathing of the solid earth seems to afford a wide field for scientific activity. It would be premature to speculate as to how far it will be possible to educe law from what is now chaotic; but it is clear that the co-operation of many observers will be required to separate the purely local from the true terrestrial changes. The directors of astronomical observatories have peculiar facilities for the study of displacements of the vertical, and it is to be regretted that hitherto most of them have been contented to banish, as far as may be, the troubles caused in their astronomical work by earth-tremors and displacements of the vertical.—Fortnightly Review.
- ↑ I make no attempt to apportion the credit among the several inventors of these instruments. The men mentioned have played the leading parts, and the work of all seems to be thorough and sound.
- ↑ "Memoirs of the Science Department of the University of Tokio," No. 9, 1883, p. 13.
- ↑ "Memoirs of the Science Department of the University of Tokio," No. 9, 1883, p. 58.
- ↑ Milne, "Earthquakes," p. 134.
- ↑ Ibid., p. 144.
- ↑ Milne, "Earthquakes," p. 189.
- ↑ Geikie, "Contemporary Review," October, 1886.
- ↑ "Report to the British Association on the Lunar Disturbances of Gravity," 1881.
- ↑ "Second Report to the British Association on Lunar Disturbances of Gravity." 1882.