THE EARLY HISTORY OF SEISMOMETRY TO 1900
James Dewey and Perry Byerly
The earliest seismoscope was invented in 132 A.D., by Chang Hêng. Seismoscopes of limited effectiveness were used by Bina and others in the eighteenth century. The middle nineteenth century saw the invention by Palmieri of a seismoscope to record the times of small earthquakes.
A successful seismograph of low sensitivity was invented by Cecchi in 1875. British scientists at the College of Engineering, Tokyo, independently built seismographs in the 1880's. The British in Japan made many observations with their instruments and must be credited with first demonstrating the value to seismology of seismographic devices.
Von Rebeur-Paschwitz obtained the first recording of a teleseism in 1889. In the next decade, investigators in Italy, Germany, and England studied the waves from distant earthquakes and constructed the first teleseismic travel-time charts. Wiechert introduced a seismometer with viscous damping in 1898.
Theory seems to have been neglected in the early development of the seismograph. Theoretical studies of forced damped harmonic-oscillator seismographs were presented by Perry and Ayrton, and Lippmann, but these had little effect on the construction of seismographs. In the 1890's, the importance of tilt was much debated. By 1900, many seismologists had become convinced that the effect of tilting on seismograph response could
Instruments have been used in the study of earthquakes for over eighteen hundred years. The earliest known seismoscope was intended to record both the occurrence of earthquakes and the azimuths of their origins from the observer. In the eighteenth century, we find proposals to record the times of earthquakes and the character of the ground motion occurring in earthquakes. A most important advance was made late in the nineteenth century, with the invention of instruments which gave records representing earthquake ground motion as a continuous function of time. We will call such instruments seismographs, regardless of the accuracy of their representations of earth motion. Our history will end at the beginning of the twentieth century.
The Earliest Seismoscope
The Chinese philosopher Chang Hêng [Chang Hêng is also referred to as Choko and Tyoko, modifications of the Japanese form of his name (Needham, 1959).] invented the earliest known seismoscope in 132 A.D. A fascinating and complete account of the seismoscope is given by Needham (1959). The instrument was said to resemble a wine jar of diameter six feet (Figure 1). On the outside of the vessel there were eight dragon-heads, facing the eight principal directions of the compass. Below each of the dragon-heads was a toad, with its mouth opened toward the dragon. The mouth of each dragon held a ball. At the occurrence of an earthquake, one of the eight dragon-mouths would release a ball into the open mouth of the toad situated below. The direction of the shaking determined which of the dragons released its ball. The instrument is reported to have detected a four-hundred-mile distant earthquake which was not felt at the location of the seismoscope.
The inside of the Chinese seismoscope is unknown. Seismologists of the nineteenth and twentieth centuries have speculated on mechanisms which would duplicate the behavior of Chang Hêng's seismoscope, but would not be beyond the Chinese technology of Chang Hêng's time. All assume the use of some kind of pendulum as the primary sensing element, the motion of which would activate one of the dragons. In his translation of the original Chinese description of Chang Hêng's seismoscope, Milne (1886b, p. 13-15) implied that the pendulum was a suspended mass, or, as we shall call it, a common pendulum. Imamura (1939) thought an inverted pendulum more probable. Hagiwara constructed an inverted-pendulum seismoscope which behaved nearly as Chang Hêng's was reported to have behaved (Imamura, 1939). The model designed by Hagiwara, however, responded most frequently to transverse motion, and indicated a direction normal to the azimuth between observer and epicenter, whereas the Chinese seismoscope was reported to have indicated the azimuth of the earthquake. Needham (1959, p. 630) has suggested that Chang Hêng's "earthquake weathercock" was calibrated empirically for its direction-determining properties.
Figure 1. Chang Heng's seismoscope, as visualized by Wang Chen-To (1936).
Needham reports that knowledge of Chang Hêng's instrument remained for over four centuries. Books describing the working of "earthquake weathercocks" were written as late as the end of the sixth century. In later years, however, the seismoscope seemed to disappear from Chinese science; later Chinese writers questioned that such a machine was possible.
Seismoscopes in Eighteenth Century Europe
We find Europeans writing on earthquake-detecting instruments from the early eighteenth century. In 1703, J. de la Haute Feuille proposed filling a bowl to the brim with mercury, so that an earthquake would cause some of the mercury to spill out (de la Haute Feuille, 1703; Favaro, 1884). In order to determine the direction of the shock, the mercury spilling out in each of the eight principal directions of the compass was to be collected in cavities or other containers. In an earthquake, de la Haute Feuille assumed, the ground would be inclined, and the direction in which the mercury spilled would be away from the origin, "where the ground began to be raised" (Favaro, 1884, p. 95). It is interesting that this earliest of European instruments was expected to respond to tilting of the Earth's surface rather than to horizontal displacements. An indication of the distance of the epicenter and the size of the disturbance could be had from the amount of mercury which had sloshed out.
An important function of de la Haute Feuille's instrument was to provide data from which future earthquakes might be predicted. His proposed method of earthquake prediction was to use his instrument to record the small shocks which he supposed must precede a large earthquake. Each day, an observer would note the amount of mercury spilled out of the bowl, if any had been spilled. If a large amount of mercury spilled out, a large earthquake would probably be imminent. With a large body of observations, patterns might be detected which would enable the prediction of future earthquakes. De la Haute Feuille seemed to believe that earthquakes were caused by the explosion of sulphurous matter and various salts within the Earth. Davison (1927) has indicated that this idea was widespread in de la Haute Feuille's time.
De La Haute Feuille clearly understood that instruments would make an important contribution to seismology. He emphasized the need for large numbers of instrumental observations in order to learn about earthquakes. Nevertheless, there is no evidence that his seismoscope was ever built. De la Haute Feuille's suggestion seems to have had little influence on the development of seismometers. A closely similar instrument was built in 1784 by A. Cavalli, probably without knowledge of de la Haute Feuille's writing (Baratta, 1895, p. .
The honor of being the first European to record the use of a mechanical device as an aid to the study of earthquakes goes to Nicholas Cirillo (1747). Cirillo employed simple pendulums in an investigation of a series of earthquakes in Naples in 1731. He observed the amplitude of pendulum oscillations at the locations where the shaking was most severe, and also at locations somewhat removed from the zone of severest shaking. He found the amplitude to decrease with the inverse square of the distance, a result he anticipated from "the common laws in other sorts of motions" (Cirillo, 1747, p. 682).
In 1751, Andrea Bina proposed suspending a common pendulum, with a pointer attached to its lower end, above a tray of fine sand (Bina, 1751). The relative motion of the pendulum bob and the Earth was to be traced in the sand by the pointer. Bina seems to have built his instrument, but we do not know if an earthquake was ever recorded with the device. Bina's instrument was intended to tell the observer the character of the ground motion. The nature of the record traced in the sand would reveal whether the earthquake motion was "regular or swaying ..., tremulous or irregular..." (Bina, 1751, p.46).
The construction of instruments in eighteenth-century Italy frequently coincided with periods of unusually high local seismic activity. The Calabrian earthquakes of 1783 were responsible for the greatest surge of interest. Davison (1936) lists six "principal" earthquakes in this series of shocks. Loss of life and property was enormous. The earthquakes were the subject of several detailed investigations, including a study by the first appointed "Earthquake Commission" (Davison, 1927, p. 29). The use of mechanical devices to verify the occurrence of earthquakes seems to have been natural to people living in areas affected by these shocks. Salfi (1787, p. 46) reported that the general populace used liquid-filled bowls and delicately-balanced objects as seismoscopes.
The shocks also spurred the invention of more elaborate instruments. The most complete treatment of these instruments is given by Baratta (1895), who reproduces important sections of the original papers. Lacking some of the original papers, we have relied heavily on Baratta's study and the portions of the originals reproduced therein.
D. Domemico Salsano, a clock-maker and mechanic of Naples, invented a "geo-sismometro" in February, 1783 (Salsano, 1783). It was operating shortly after the first large Calabrian earthquake. It was a common pendulum, eight and a half "parisian" feet long. The pendulum mass was equipped with a brush, which was to record the motion of the mass with slow-drying ink on an ivory slab.
Reports of the observations made by Salsano (Salsano, 1783; Torcia, 1784) suggest that his pendulum may have responded to some of the earthquakes in Calabria, about two hundred miles away. The nature of the recorded observations suggests that many motions of the pendulum were not due to earthquakes, as might be expected for a pendulum in an unsheltered location. Nevertheless, sometimes the pendulum motions were roughly contemporaneous with earthquakes in Calabria. We are not told how exactly the pendulum motion and the earthquakes coincided in time. It is clear also from the description of observations that most, it not all, of these observations were made by watching the pendulum, rather than by examing records written by the pendulum. The pendulum was arranged to ring a bell when its oscillations were large enough. This it did on several occasions.
Salsano's seismoscope received relatively little mention in the large studies of the Calabrian earthquakes. Baratta notes that some who did refer to the machine treated it with scorn. One writer, F. Salfi, pointed out that the instrument was nothing more than an ordinary pendulum, and criticized those who would "push to the foot of the throne" the merits of its inventor (Salfi, 1787, p. 46). Salfi's statement, however, hints that Salsano's instrument was well known in its time, in spite of the lack of printed recognition. Later, during the New Madrid earthquakes of 1811 and 1812 in America, Daniel Drake of Cincinnatti reported the used of an "instrument constructed on the principle of that used in Naples, at the time of the memorable Calabrian earthquake" which reportedly "marked the direction of undulation from south-southwest to north-northeast" (Fuller, 1912, p.27). Baratta (1895) also mentions a common pendulum constructed by Zupo, which wrote in sand. Perhaps this was the Neapolitan instrument to which Drake referred.
In the years immediately following the Calabrian earthquakes, two other seismic instruments were described which should be mentioned here. A. Cavalli, in 1784, reinvented de la Haute Feuille's mercury-filled-bowl seismoscope (Cavalli, 1785). In addition, he designed a modification of this instrument which would give the time of an earthquake, to the nearest minute. This was to be accomplished by the use of platforms rotating beneath two mercury-filled bowls. As the platforms rotated, cavities corresponding respectively to the hour of the day and the minute of the hour would pass beneath the notches in the sides of the bowls. When mercury overflowed from the bowls through the notches, it would be conveyed into the two cavities corresponding to the hour and minute of the day. The observations reported by Cavalli suggest that the time-telling part of his seismoscope was never built (Cavalli, 1785). This instrument, if it was constructed, was the first designed to tell the time of an earthquake.
The Duca della Torre, A. Filomarino, invented a common pendulum "sismografo", similar to Salsano's, but with the addition of a time-telling device. The first known description of this instrument is from 1796 (della Torre, 1796), although Salsano (1783) mentioned an instrument similar to his own, which he had not seen, invented contemporaneously by Duca della Torre. A record was to be written by a pencil attached to the pendulum and pressed gently with a spring against a piece of paper. On the pendulum mass was to be put a hair, which would arrest the balance wheel of a clock. When the mass moved, the hair would be withdrawn and the clock would start.
Several written records were observed with this instrument. The records consisted of a pencil-line or two for each earthquake. They must have contained little more information about earthquake motion than could have been obtained from "natural seismograms", such as scratches left by a heavy object moving on a smooth surface. Nevertheless, the Duca della Torre is the first to describe a record obtained with an instrument constructed as a seismometer. The observations given by him, as reproduced by Baratta (1895, p. 31) do not suggest that the timing device was functioning.
The "Seismometer" of James Forbes
The interest in seismic instruments generated by the Calabrian earthquakes was not sustained in succeeding years. In 1839, however, a series of small earthquakes began near Comrie, in Perthshire, Scotland. Scores of shocks were felt over a period of several years. A direct result of the Comrie earthquakes was the establishment of a Special Committee of the British Association for the Advancement of Science, the purpose of the committee being to obtain "instruments and registers to record shocks in Great Britain" (Milne, 1842).
The most significant instrument resulting from the committee's work was an inverted-pendulum "seismometer", designed by James Forbes (Forbes, 1844). The seismometer is shown in Figure 2. It consisted of a vertical metal rod having a mass C moveable upon it. The rod was supported on a vertical cylindrical steel wire. The wire could be made more or less stiff by pinching it at a greater or lesser height by means of a screw S. By adjusting the stiffness of the wire, or the height of the ball, the free period of the pendulum might be altered. A pencil L placed on the prolongation of the metal rod wrote a record on a stationary, paper-lined, spherical dome I. By placing the pencil sufficiently far above the mass, a magnification of the motion of the mass by a factor of two or three could be obtained.
Figure 2. Forbes' seismometer (after Forbes, 1844). The screws E, acting on the support D, are used to help set the pendulum in an upright position.
Forbes used the inverted pendulum, mounted on a stiff wire, to provide a sensing element in weakly-stable equilibrium, without having to use a very long common pendulum. An upright inverted pendulum, by itself, is in unstable equilibrium. A slight motion topples it over. When the inverted pendulum is mounted on a suitably stiff wire, the apparatus may be rendered stable, so that the inverted pendulum returns to an upright position after having been disturbed. Such a combination had been used already by a Mr. Hardy for the purpose of detecting vibrations set up in a clock frame by the beating of the clock (Kater, 1818).
Forbes was probably the first to attempt explicitly to give a seismological instrument a "long" period. [Forbes speaks of requiring, for earthquake recording, a common pendulum, ten or twenty feet long, which would have a period of four or five seconds (Forbes, 1844, p. 219). Later, in a discussion of the size of his seismometer, he states that the sensibility of seismometers of different size should be the same - "say one second" (Forbes, 1844, p. 221). We suspect, however, that he hoped for a period nearer five seconds than one second, since his seismometer offered no advantage in compactness over a common pendulum with a one-second period. We have found no indication of the period actually obtained with the instrument.] He was also the first to try to avoid the clumsiness of a long common pendulum in obtaining long periods. As we shall see, his method of approaching neutral equilibrium was similar to that used by Wiechert in 1900. Forbes desired a long period in order that the pendulum remain stationary as the Earth moved beneath it. He clearly wanted to measure ground displacement in an earthquake. However, he considered only the effect on his instrument of an "earthquake" consisting of a single, horizontal displacement of uniform velocity, beginning and ending suddenly. For this reason, we can't be sure if Forbes knew that a long-period pendulum would function as a displacement meter for very-short-period oscillations of the ground. For the type of motion he considered, Forbes expected his seismometer to show a straight line, corresponding to the sudden displacement of the Earth, which would be easily distinguished from the ellipsoidal traces caused by the pendulum oscillating about its new equilibrium position.
A mathematical theory of the instrument accompanies its description (Forbes, 1844). Forbes was the first to describe mathematically the behavior of a seismic instrument in an "earthquake". The assumed earthquake, again, is a single motion of uniform velocity, starting and stopping abruptly.
Six of the inverted-pendulum seismometers were set up at Comrie (Milne, 1842, 1843). One was thirty-nine inches long and another was ten feet long. The lengths of the remaining four were not noted. Neither were the periods of the instruments noted. They gave disappointing performances. In one year, for example, the seismometers recorded only three of sixty earthquakes felt at Comrie (Milne, 1843). The nature of the records is difficult to decipher from the descriptions given by the investigators, who mention the indicating pencil being displaced by the quake, but don't mention any records being written by it. They conclude that the motion in the recorded earthquakes consisted of a sudden movement of the ground, with a maximum displacement of one half of an inch horizontally (Milne, 1842, 1843).
Friction between the writing pencil and the recording surface must take some of the blame for the disappointing results given by the seismometer. Forbes was aware of the problem of friction. He attempted to overcome it by using a heavy mass, but he does not seem to have been successful. We will see that friction would severely limit the sensitivity of many later seismographs which had mechanical registration.
Common pendulums and an instrument for measuring vertical motion were also used at Comrie. The vertical motion seismoscope consisted of a horizontal metal bar, loaded with a weight at one end, and fastened to the wall with a flat spring. In an earthquake, the weight was expected to lag behind the ground motion and move a light straw to indicate the extent of vertical motion. The vertical-motion instrument functioned on several occasions, indicating vertical "displacements" as great as half an inch.
The earthquakes at Comrie diminished in number, and after several years the committee on earthquake instruments was allowed to dissolve.
Further Studies with Seismoscopes
The years between 1850 and 1870 saw several significant contributions to seismological instrumentation. These included Palmieri's seismoscope for recording the time of an earthquake, and a suggestion by Zöllner that the horizontal pendulum might be used in a seismometer. Mallet studied earthquake motion by observing the effects of earthquakes and by measuring the velocity of elastic waves generated by explosions. A true seismograph still eluded seismologists.
Figure 3. Mallet's seismoscope (after Mallet, 1852). The image of a cross-hairs in C is reflected from the surface of mercury in the basin B and viewed through a magnifier, D.
Explosion seismology was born in 1851, when Robert Mallet used dynamite explosions to measure the speed of elastic waves in surface rocks (Mallet, 1852, 1862a). He wished to obtain approximate values for the velocities with which earthquake waves were likely to travel. To detect the waves from the explosions, Mallet looked through an eleven-power magnifier at the image of a cross-hairs reflected in the surface of mercury in a container (see Figure 3). A slight shaking caused the image to blur or disappear. Transit velocities were measured over distances of the order of a thousand feet. For granite, Mallet obtained velocities of about 1600 feet per second. He had expected to find velocities of 8000 feet per second. The unexpectedly low elastic-wave velocity was attributed to the heterogeneity of the rock through which the wave traveled. Later investigators (Abbot, 1878; Fouqué and Levy, 1888), using instruments similar to Mallet's, have found higher velocities and have suggested that Mallet may not have detected the earliest arrivals in his experiments.
Mallet advocated the use of fallen objects and cracks in buildings as aids in the study of earthquakes. He made a detailed investigation of the Neapolitan earthquake of 1857, in which he paid particular attention to the way buildings were cracked, walls overthrown, and soft ground fissured (Mallet, 1862b). Mallet believed that an earthquake consisted primarily of a compression followed by a dilatation. For such a shaking, he suggested, the resulting cracks in structures would be transverse to the direction of wave propagation. Overturned objects would fall along the horizontal projection of the direction of wave propagation. By observing the directions of arrival from a number of different points, he plotted an origin from which the wave seemed to spread. Mallet also published a set of formulae for calculating the velocities necessary to overturn structures of various simple shapes. From these, and observations of overturned objects, he estimated the velocity of particle motion at different sites.
Mallet's assumption that earthquakes consisted mainly of longitudinal motion was proven invalid as soon as seismometers were built which recorded the large transverse component of ground motion. Interest remained in the possibility of using overturned columns and other ruins as seismoscopes for measuring certain parameters of earthquakes, such as particle velocity and acceleration. In the 1880's, investigators in Japan checked Mallet's method of calculating ground velocity from observations of bodies overturned or displaced by earthquake motion. Velocities calculated from Mallet's formulae did not in general agree with those calculated from seismograms (Milne, 1885b). J. Milne and F. Omori introduced a formula for the acceleration necessary to overturn columns, which they checked by putting columns on a cart and shaking them with a sinusoidal motion (Milne and Omori, 1893). Omori applied the formula to gravestones overturned in the epicentral region of the Mino-Owari earthquake of October 28, 1891 (Milne, 1893c). He found accelerations greater than 0.4 g.
A design of a pendulum seismoscope was reported by Kreil (1855). In this instrument, the pendulum mass was to be a cylinder, on which recording paper was to be wrapped. The recording stylus, fixed to the ground, would write on the pendulum mass as it moved in an earthquake. The mass was to be rotated by a clock at a rate of once every twenty-four hours. In this way, the time of an earthquake could be noted. There is no evidence that Kreil's machine was built, although its design seems to have been considered significant by seismologists of the day (Mallet, 1859, p. 76).
In 1858, P. G. M. Cavalleri (1858, 1860) reported the construction of a common-pendulum seismometer similar to that described by Bina, more than one hundred years earlier. As in Bina's instrument, a pointer on the pendulum bob traced a record of the motion of the pendulum in fine powder. Observations of felt earthquakes suggest to Calvalleri that the frequency of earthquake waves would be three cycles per second. For this frequency, the 1.25 meter-long common pendulum would function approximately as a displacement meter, as its inventor intended.
Cavalleri described other instruments. A mass on a spiral spring was intended to detect vertical motion. It had a period of one second - long enough, Cavalleri felt, to record the rapid pulse-like vertical displacement of the ground believed to occur in an earthquake. The mass of this instrument was connected to the short arm of an indicating lever. The lever was constructed so that it would remain at the position of its maximum excursion.
Finally, in order to study the frequency content of earthquake waves, Cavalleri constructed six short pendulums of different periods, each of which traced the record of its motion in fine powder, as the larger pendulum did. Assuming that a range of frequencies from two to four cycles per second would be "sufficient to embrace every undulation occasioned by any earthquake" (Cavalleri, 1860, p. 113), Cavalleri expected that the pendulum whose period was closed to the predominant period of the earthquake would resonate and show a larger amplitude than the other pendulums. This apparatus was new to Europeans. Jared Brooks of Louisville, Kentucky, had constructed pendulums of different lengths to observe the New Madrid earthquakes of 1811 and 1812 (Fuller, 1912, p. 32).
In 1856, Luigi Palmieri installed his "sismografo elettro-magnetico" in the volcanic observatory on Mount Vesuvius (Palmieri, 1871, 1874). This instrument was intended to give the direction, intensity, and duration of an earthquake, and was capable of responding to both horizontal and vertical motions. It was not a "seismograph" in the sense in which we are using the word, but rather a collection of seismoscopes, each intended to record particular parameters of an earthquake (Figure 4).
Figure 4. Palmieri's "sismografo elettro-magnetico" (reproduced from The Engineer, 33, 1877, p. 407). Vertical motion is detected by a mass on a spiral spring E. The U-tubes n detect horizontal motion. Paper is unrolled from the drum i and a pencil mark put on the paper at m. The speed of the paper is regulated by the clock B. The clock A is stopped by the earthquake to give the time of the shock.
The seismoscope for detecting vertical motion consisted of a conical mass on a spiral spring. The mass was suspended just over a basin of mercury. When a slight motion caused the tip of the cone to touch the mercury, an electric circuit was completed, which caused a clock to stop, indicating the time of the shock. The spiral spring was constructed so that thermal changes in the length of the spring were balanced by thermal changes in the length of the frame to which the spring was attached.
Horizontal motion was detected with common pendulums, whose swinging completed the same circuit as that completed by the mass-spring seismoscope. In addition, U-tubes filled with mercury were used to detect horizontal motion.
The closing of the above-mentioned electric circuit, besides stopping the clock, started a paper recording surface and caused a pencil to be pressed against the surface. The recorder, once started, continued running until the paper was used up. Every time the circuit was completed, a pencil dash would be left on the moving paper. The duration of the quake was thereby recorded. The size of the earthquake was indicated by the amplitude of oscillations suffered by a mass on a spring and by the amplitudes of the oscillations of the mercury in the U-tubes. The size of the earthquake was measured in "degrees".
Palmieri's "sismografo" seems to have been an effective earthquake detector for its time. Palmieri used it for many years on Mount Vesuvius and detected numerous shocks with the instrument (Palmieri, 1862a, 1862b, 1864, 1866, 1867, 1869, 1870, 1876). Disturbingly, however, the instrument was unable to detect many shocks which were felt in the nearby city of Naples. Palmieri, in fact, believed that the apparatus functioned better as a predictor of earthquakes and volcanic eruptions. He observed that many instances before the eruption of Vesuvius or other Mediterranean volcanoes, or before large earthquakes in the Mediterranean area, his seismoscope would detect "shocks". But when the earthquakes occurred, even if they were felt in Naples, the instrument would not detect them. (Fortunately, an identical instrument located in Naples did detect the earthquakes felt there.) Palmieri observed that before Vesuvius was going to erupt, the "shocks are more frequent; or to express it better, the ground trembles in a continuous manner with diverse phases" (Palmieri, 1867).
Palmieri's "sismografo" was later used by seismologists in Japan. For ten years it was used to detect earthquakes in Tokyo; 565 earthquakes were detected from October, 1875 to March, 1885 (Milne, 1880c, 1883b, 1885c). For most of these earthquakes, "force" (the size of the earthquake in "degrees") and direction of motion, as well as time, are catalogued, indicating that the whole "sismografo" was functioning. It should be noted that some workers in Japan did not believe the "force" to be necessarily even an approximate indication of the relative "intensity" of different earthquakes (see, for example, Ewing, 1883a, p. 72).
After 1885, routine earthquake recording in Tokyo was taken over by the new seismographs just developed in Japan, but Palmieri's circuit-closing seismoscopes were used past the turn of the century as triggering devices to start recording systems in other seismographs (Holden, 1898). One such seismoscope was among the seismographic equipment at Mount Hamilton, California, at the time of the California earthquake of April 18, 1906. The seismoscope did not trigger the other seismographs until thirty-three seconds after the first tremors were felt at Mount Hamilton (Read, 1910, p. 64).
The horizontal pendulum appears to have been independently invented several times in the nineteenth century (Darwin, 1882; Davison, 1896). In 1869, Zöllner described a horizontal pendulum with the suspension which has since been associated with his name (Zöllner, 1869, 1872). Zöllner's suspension is shown in Figure 5. The rod R, with a mass on one end, rotates about the axis AC, which is inclined at an angle i to the vertical, V. The rod is supported by wires AB and CD, which are attached to the rod at points B and D some distance from each other. A mirror on the pendulum was used to reflect a light beam from a lamp to a scale, where the motion of the pendulum, as magnified by the optical lever, was directly observed. The instrument was installed in the cellar of the university in Leipzig; Zöller could detect significant movement of the pendulum due to the filling up of the auditorium on the second floor of the building. The pendulum was built in order to observe changes in the direction of gravity due to tidal forces, but Zöllner suggested that it might also be valuable as a seismometer.
Figure 5. Zöllner's horizontal-pendulum suspension.
Horizontal pendulums were to be widely used in seismographs after 1880, because they could be given long periods and could still be compact. There were several different suspensions used in these later horizontal-pendulum instruments. The Zöllner suspension was used in the Galitzin horizontal seismograph. The suspension of the Wood-Anderson torsion seismometer may be considered a limiting case of the Zöllner suspension.
The Invention of a Seismograph in Italy
The research begun by Cavalleri and Palmieri was vigorously continued by Italian seismologists in the 1870's. 1874 saw the publication of the first journal devoted to solid-earth geophysics, the Bulletino del Vulcanismo Italiano, founded and edited by M. S. De Rossi. In spite of the high seismological activity, however, relatively few instrumental advances emerged from this period, and the instrument which to us is most significant seems to have been little noticed at the time it was invented. This was Cecchi's seismograph, which we will describe in the next paragraphs. The great majority of the other instruments designed by Italian seismologists at this time were seismoscopes of the same general type as had been invented earlier by Palmieri.
Cecchi's instrument has been described by later Italian seismologists as the first true seismograph (Agamennone, 1906, p. 91). The machine was apparently built in 1875 (Cecchi, 1876). Unlike any of the instruments we have discussed so far, the Cecchi seismograph was expected to record the relative motion of a pendulum and the Earth as a function of time. For horizontal vibrations, two common pendulums were used, one vibrating in a north-south plane and the other vibrating in an east-west plane. The pendulums "beat seconds"; their motion was magnified three times by a thread-and-pulley apparatus. For vertical motions, a mass on a spiral spring was used. Finally, a machine which was expected to record rotary motions, was incorporated into the seismograph. This consisted of a cross bar with weights at both ends, much like a dumbbell, which was pivoted at its center of mass so as to rotate in a horizontal plane. Restoring force was applied to the dumbbell by springs, so that it oscillated with a period of one second.
Cecchi arranged a seismoscope to start a clock and to start into motion the recording surface at the time of an earthquake. The recording surface would translate under the indicating needles at a speed of one centimeter-per-second for twenty seconds. From the time on the clock, an observer arriving at the seismograph would determine how long before his arrival the earthquake had occurred.
Unfortunately, the subsequent history of Cecchi's earliest seismograph is unclear. A modified form of the instrument was reported to have been installed in Manila (Du Bois, 1885). This instrument and the later Cecchi seismographs reported by Agamennone (1906) are not so interesting to us because, by the time these instruments were built, better seismographs were being used by British scientists in Japan. The early form of the Cecchi instrument was apparently installed in several observatories. One might think that one of these seismographs would have recorded an earthquake in the five years between 1875 and 1880, the latter being the year of the earliest seismogram obtained by the British in Japan, who also claimed the first real seismograph. But the earliest date we have found for a seismogram obtained with the early Cecchi instrument is February 23, 1887, when one of these seismographs recorded a large earthquake which occurred in the French-Italian border region (Denza, 1887). In that earthquake, only the east-west component was recorded, although the seismograph was located in a zone of strong shaking. The instrument must have been most insensitive. The seismogram is reproduced in Figure 6. The record appears to us to be as accurate a representation of earth movement as was obtained by the early seismographs in Japan. Assuming that the recording instrument had not been significantly altered between 1875 and 1887, it would seem that Cecchi indeed deserves credit for the construction of the earliest seismograph, insensitive though it might have been.
Figure 6. The record obtained by a Cecchi seismograph at Moncalieri, Italy, on February 23, 1887 (reproduced from Fouqué, Tremblements de Terre, Bailliére, p. 79).
Cecchi's seismograph had relatively little impact upon Italian seismology. The insensitivity of the instrument must have discouraged others from building seismographic devices (De Rossi, 1887). In addition, Cecchi's apparatus would have been costly and unsuitable for an observatory of modest means. Finally, many Italian seismologists seemed to believe that an earthquake could be satisfactorily described by seismoscopic data (De Rossi, 1877, p. 9). We won't discuss in detail any more of the Italian seismoscopes. Descriptions of many are given by Agamennone (1906) and Ehlert (1897a). A review in English of the Italian instruments was given in The Electrical World (anon., 1887). Some of these seismoscopes depended on the fall of a delicately-balanced object to trip a time-recording device and sound an alarm. Many were modifications of Palmieri's seismoscopes. The microphone was used by some Italian seismologists to listen to "earth noises" (De Rossi, 1883).
An example of the use of seismoscopes in Europe is the experiment of von Lasaulx (Hoernes, 1893). He constructed an apparatus in which the fall of a poised weight caused a pendulum clock to be stopped. About one hundred and fifty of these devices were installed at telegraph stations in Germany. The sensitivity of the installed seismoscopes was deliberately kept low, so that human disturbances would not cause the clocks to be stopped frequently. Nevertheless, two earthquakes were large enough to be detected by many of the seismoscopes. Time and "direction of motion" were determined from each instrument. The data were contradictory and seemed only to indicate that most of the clocks at the telegraph stations were not sufficiently accurate for seismological work.
The Development of the Seismograph in Japan
Cecchi's seismograph notwithstanding, it seems clear to us that the credit for the introduction of the seismograph as an essential tool in seismology belongs to a group of British professors teaching in Japan in the late nineteenth century. These scientists obtained the first known records of ground motion as a function of time. Furthermore, they knew what such records could reveal about the nature of earthquake motion. They used their instruments to study the propagation of seismic waves, and they used them to study, for engineering purposes, the behavior of the ground in earthquakes.
The principal figure among the British seismologists in Japan was John Milne. It has been noted (Davison, 1927) that Milne's influence on seismology extends far beyond his own contributions, which were themselves considerable. The Seismological Society of Japan was founded at Milne's urging, in the spring of l880, following a sharp earthquake in Yokahama. The society was the first devoted to seismology; its founding marked the beginning of a period of rapid growth of seismology in Japan. Nearly two decades later, Milne was to be largely responsible for having similar seismographs set up at stations throughout the world, in order to collect data which could be evaluated at a central observatory. Milne's own investigations were rather more observational than theoretical. Although Milne devised several significant instruments, his primary importance to this paper is in the use to which he put both his own and others' instruments.
We will frequently cite the work of James Ewing and Thomas Gray, who designed the most important of the early instruments, in the years immediately following the founding of the society. Unfortunately, their stay in Japan was short; Gray left in 1881 and Ewing in 1883 (Davison, 1927). It was left largely to Milne to show the value to seismology of the Gray and Ewing seismographs. All three, Milne, Ewing and Gray, were visiting professors at the Imperial College of Engineering in Tokyo. The membership lists of the Seismological Society of Japan, in fact, reveal a greater number of foreign members than Japanese members (Seismological Society of Japan, 1880), and the instrumental innovations of the 1880's were due almost entirely to the visiting British professors.
In the decade preceding 1880, several instruments were constructed in Japan which deserve consideration although they did not function successfully as seismographs. Verbeck made the first pendulum observations of earthquakes in Japan in 1873 (Milne, 1880a). Wagener (1880) constructed a common-pendulum seismometer which successfully detected earthquakes, but apparently it did not write records.
Figure 7. Wagener's pendulum seismoscope (after Ehlert, 1897a).
Wagener's machine is shown in Figure 7. The motion of the pendulum bob M was magnified by a mechanical lever, T. The fulcrum B of the lever was attached to the ground, while the short arm of the lever was attached to the base of the pendulum bob. The displacement of the bob relative to the ground was magnified twenty-four times at the end C of the long arm of the lever. To this end of the lever was attached a piece of thread, which was, in turn, wrapped around a spool R. As the lever moved, the thread pulled off the spool, thereby turning the spool. An indicating needle, I, turned with the spool to indicate the relative size of the quake. The pulling of the thread also stopped a clock, thus giving the time of the earthquake. Wagener also built an apparatus which determined the azimuth of the earthquake motion and the amplitude of vertical motion.
In a seventeen-month period, in 1878, 1879 and 1880, in which Wagener's instrument was in operation, it detected twenty-seven earthquakes. The instrument seems to have been set up in the vicinity of Tokyo Bay (Wagener, 1880). Most of the recorded earthquakes were shown to have maximum ground-displacements of less than a millimeter, although two were indicated to have had a displacement greater than 2.5 millimeters. The observer reported that "the extent of the motion as given by the indicator has always been in perfect accordance with the violence of the shock, as far as this can be estimated by the feeling" (Wagener, 1880, p. 71). The vertical motion instrument detected vertical displacement in only eight of the twenty-seven events.
Wagener planned to record the motion of the indicator needle continuously on a translating, rotating drum. He does not appear to have carried out this part of his plan (Ewing, 1883a, p. 38). As it was constructed, then, Wagener's machine still did not constitute a seismograph. It did not write a record, but only indicated the maximum horizontal shaking.
W. S. Chaplin constructed a horizontal-pendulum seismometer in Japan in 1878 (Ewing, 1883a, p. 21). James Ewing described it as a "wooden rod, free to turn about a vertical axis, and carrying at its end a rigidly attached block". The instrument did not work, a failure which Ewing attributed to either low magnification or excessive friction at the pivot points. Chaplin's instrument is important as the first attempt to build a seismometer with a horizontal pendulum as a sensing element.
A most interesting and important paper, entitled "On a neglected Principle that may be employed in Earthquake Measurements" was presented in 1877 to the Asiatic Society of Japan by J. Perry and W. E. Ayrton (1879). The authors analyzed the motion of damped and undamped mass-spring oscillators subjected to a periodic force. The relative displacement of the oscillator mass and the ground is expressible as a sum of steady-state terms and transient terms. To minimize the effect of the transient terms, Perry and Ayrton recommended using an oscillator with a period much shorter than the periods expected in an earthquake. That is, they recommended an accelerometer for the most accurate recording of earthquakes. They recognized that the relative displacement of the mass and the earth, with such an instrument, would be less than the actual displacement of the Earth, but they don't seem to have thought that this would prevent the apparatus from obtaining useable records. Perry and Ayrton also considered oscillators with periods greater than or equal to the periods of earthquake motion. They pointed out that these oscillators, when damped with viscous material, could also be used to obtain reasonably accurate records of earthquake motion.
Figure 8. Ewing's common-pendulum seismograph (after Ewing, 1883a). Shown in cross-section are the ring-shaped bob A spanned by the cross-bar C. Attached to the middle of the cross-bar is a brass plate, L, which has a slot in it. The lever M rests loosely in the slot. Motion perpendicular to the slot is transferred to M and writes on the recording surface S. For motion parallel to the slot, M slides in the slot and is not affected. Another lever, not shown, records the component of horizontal motion parallel to the slot. In this way, the horizontal ground motion is resolved into two perpendicular components.
Perry and Ayrton designed an instrument to have their recommended short period. It was to consist of a heavy spherical weight, free to move horizontally and vertically, and held in equilibrium by five springs. The motion of the ball was to be resolved into three rectilinear components and written on a moving band of paper. This seismograph was never built. James Ewing (1883a, p. 69-70) criticized the design of the instrument on account of the probable difficulty of its construction. Ewing concluded, "... it is not remarkable that the principle on which it is based should, even after attention was directed to it, have remained neglected."
In rejecting Perry and Ayrton's proposed short-period instrument, contemporary seismologists seem also to have rejected the suggestion that the records of longer-period seismometers would be more accurate representations of the Earth's motions if these seismometers were damped with a viscous fluid. The authors had recommended the use of material of high viscosity, such as a mixture of tar and pitch. Perhaps this suggestion hindered acceptance of their proposal. We shall see that artificial solid friction was tried as a means of damping a seismometer. Not until the turn of the century was viscous damping introduced into seismology, with much success, by Wiechert.
It is time now to consider the work of John Milne, James Ewing, and Thomas Gray in developing the first successful seismographs in Japan. [An interesting series of letters by Milne, Gray, and Ewing in Nature (1886-1887) discusses the question of priority for the invention of certain of these seismographs. It would appear that very few of the instruments can be considered the invention of one of the men exclusively.]
Ewing's first seismograph (Figure used a twenty-one foot long common pendulum as a sensing element (Ewing, 1880a). Ewing anticipated that the pendulum's five-second period would be sufficiently long that the bob would remain stationary under the short pulse-like motions of the ground which he believed occurred in earthquakes. He took care to build a rigid frame for the pendulum, so that the motion of the frame would not contribute spurious oscillations on the record.
The relative motion of the pendulum bob and the ground was magnified six times and recorded on a continuously-revolving, circular smoked-glass plate. The magnification was achieved with a simple lever whose fulcrum was attached to the ground. The short arm of the lever was attached to the pendulum bob and the long arm was the indicating lever, scratching a trace on the smoked-glass plate. Expecting that force due to the friction of the indicating lever would be transmitted back to the bob, Ewing attached the lever to the center of mass of the bob, so that this frictional force would not exert a torque on the bob and cause the bob to rotate or wobble. The motion of the pendulum bob could be resolved into two perpendicular components by the recording apparatus.
Figure 9. Ewing's horizontal-pendulum seismometer (after Ewing, 1880b).
Ewing's common-pendulum seismograph was constructed in 1879. This seismograph was not operated continuously from its birth. It therefore did not record its first earthquake until over a year later. At that time, a more sophisticated instrument, Ewing's horizontal-pendulum seismograph, was also in operation. The long common pendulum thus did not have a major influence in the development of seismology by the British in Japan. As we shall see, however, very long common pendulums were later built by Italian seismologists, and these played a significant role in the history of seismology.
Ewing's was the first successful attempt to use a horizontal pendulum to detect earthquakes (Ewing, 1880b, 1881c). Ewing's suspension is illustrated in Figure 9. A light rigid frame F is pivoted at points A and B so as to swing like a garden gate around the axis of rotation AB. At the axis of percussion of the frame is pivoted the cylindrical mass M. [The mass was pivoted on a light frame in order to obtain the maximum inertia for a given total mass and a given distance of the axis of percussion from the axis of rotation (Ewing, 1883a, p. 17). Milne (1898a, p. 138) also pivoted the masses of some of his horizontal pendulums.]
Figure 10. A record, obtained with Ewing's horizontal-pendulum seismograph, of a strong local earthquake on March 8, 1881 (reproduced from Nature, 30, 1884, p. 174). Two of the pendulums write on the same surface. The recording plate revolves continuously with one revolution every fifty seconds. The "beginning" of the earthquake is marked as a, a' respectively on the EW and NS traces. In order that the indicator pens not interfere with each other, the EW trace is put on the record at a point approximately ninety degrees clockwise from where the NS trace is put on the plate. In the center, the traces have been aligned on a common time scale.
The motion of the horizontal pendulum was magnified by means of a long indicator fastened to the frame of the pendulum. A record was inscribed on a revolving smoked-glass plate (Figure 10). There was no way to separate the signal written in one revolution from the signal written in another revolution. It was expected that the indicator would trace and retrace over the same circle on the revolving record during periods of quiet and, during an earthquake, would yield a signal superimposed on the circle. In practice, because the zero-line drifted, later versions of the instrument used a seismoscope, such as the one invented by Palmieri, to start the recording surface into motion at the time of an earthquake. In some instruments, a short pendulum was released in an earthquake to put time marks on the record. Otherwise, the relative arrival times of different phases were determined from the known speed of the recording surface.
Figure 11. "Shake table" test of Ewing's horizontal seismograph (reproduced from Memoires of the Science Dept., Univ. of Tokyo, no. 9, plate XXII). a represents the motion of the "table"; b the motion of the table as it was recorded by the seismograph. A time scale is not given.
With few exceptions, the seismographs built by the British in Japan were intended to have periods longer than the periods of the waves occurring in earthquakes, so that the seismographs would function as displacement recorders. In order to test the accuracy of his horizontal-pendulum seismograph as a displacement recorder, Ewing subjected it to a "shake-table" test (Ewing 1881c, 1883a, p. 86). Two identical instruments were placed side by side on a table, the bob of one being clamped to a nearby wall, so that it would not move, and the bob of the other being left free. The table was then shaken. It was found that the records written by both instruments were nearly identical (Figure 11), showing that the bob of the unclamped seismograph had remained nearly stationary during the shaking. The "frequency" of the shaking motion was not reported.
Both of Ewing's instruments, the common-pendulum and horizontal-pendulum seismographs, recorded a small earthquake on November 3, 1880, giving the first lengthy seismograph records of earthquake motion as a function of time. [Milne (1880b) obtained a short seismogram, three seconds in length, of the Yokohama earthquake of February 22, 1880. He recorded the motion of a long common pendulum on a moving plate.] Assuming that the seismometer functioned as a displacement meter, and assuming harmonic motion, Ewing calculated the maximum displacement (0.29 mm.) and maximum acceleration (1.6 cm/sec-sec) of the ground (Ewing, 1881a, 1883a, p. 54-55).
In his report on this and four other small earthquakes recorded in the same month, Ewing noted the most striking features of these early seismograms. They were: "(1) The very gradual beginning and ending of the disturbance. In none of the observations did the maximum motion occur until after several complete oscillations had taken place. (2) The irregularity of the motion. The successive undulations are widely different both in extent and in periodic time. (3) The large number of undulations in a single earthquake, and the continuous character of the shock. (4) The extreme minuteness of the motion at the Earth's surface" (Ewing, 1883a, p. 55). The importance of these observations can be imagined. For the first time, scientists had a representation of earthquake motion, and this representation revealed a much different manner of shaking than that which had been previously thought probable. Specifically, Robert Mallet's widely accepted view that an earthquake consisted primarily of a longitudinal pulse was shown to be incorrect. Ewing (1881b) realized that his observations were fatal to methods of determining the "velocity of the earthquake" from the times shown on stopped clocks (as had been attempted by von Lasaulx, cited above). For the first time, also, seismologists could design their instruments with some knowledge of the phenomena the instruments were to record.
A seismograph for recording vertical motion was still to be desired. Thomas Gray (1882) introduced a method of increasing the period of a mass-spring system by attaching the spiral spring to the short arm of a lever and attaching the mass to the long arm of the lever. This increased the period by increasing the effective mass of the pendulum bob. It was, however, desirable to diminish further the tendency of the mass to return to its zero position. Accordingly, Gray attached to the bob a container which was connected by a siphon to an external reservoir of mercury. When the bob was deflected downward, mercury would flow from the reservoir into the container on the bob, thus increasing the weight of the bob and neutralizing the increased restoring force of the spring. When the bob was deflected upward, mercury flowed from the container to the external reservoir, lightening the bob and, again, neutralizing the tendency of the bob to return to its equilibrium position.
James Ewing (1882a) suggested that the period of Gray's seismometer could also be increased by attaching the spring to the lever arm at a point below the line connecting the center of mass of the pendulum and the pivot of the lever (Figure 12). This gives the moment arm, to which the spring is applied, a first-order dependence on the boom deflection, with the moment arm always changing so as to decrease the restoring moment of the pendulum. This particular suspension was adopted in Japan, and was to see widespread use in other vertical-motion instruments before the introduction of the zero-length spring. Gray (1887b) and Grablovitz (1891, 1896a) introduced other suspensions which also had the effect of decreasing the restoring moment acting on the lever, when the lever was displaced.
The changing length of the spring in the Gray-Ewing vertical seismometer caused considerable drift. In the 1882 instrument, the seismometer recorded on a revolving glass plate which, because of the drift, was not run continuously. A Palmieri-type seismoscope was used to put the recording surface into motion in an earthquake.
James Ewing, in 1882, joined a common pendulum with an inverted pendulum, so that the two would move horizontally together (Ewing, 1883b, 1886). He obtained thus a system in nearly neutral equilibrium, with the unstable tendency of the inverted pendulum lessening the stability of the common pendulum to which it was attached. This was Ewing's "duplex-pendulum" seismometer (Figure 13).
The duplex-pendulum seismometer is of particular interest to us because in 1887 and 1888 this type of seismometer was placed at ten sites in Northern California and Nevada (Louderback, 1942). The first seismographic observatories in the Western Hemisphere, at Berkeley and Mount Hamilton, were equipped with the duplex instruments as well as Ewing horizontal-pendulum seismometers and Gray-Ewing vertical seismometers. The duplex seismometers in use here wrote two-dimensional particle-motion diagrams on a stationary, smoked glass plate.
Figure 12. Ewing's suspension for detecting vertical motion (after Ewing, 1882a). The spring is attached below the line between the pendulum's center of mass and the pivot point, B.
We must introduce here yet another form of the horizontal pendulum, the so-called "conical pendulum", invented by Gerard in 1851 (Gerard, 1853; Davison, 1896), and introduced to seismology by Thomas Gray (1881). In this suspension, the nearly horizontal boom is held up by a flexible wire and pivots about the axis of rotation on a bearing joint. In order to lessen the friction at the pivot point, James Ewing (1883a, p. 27) forked the boom and extended it beyond the axis of rotation (Figure 14). From the end of the fork, Ewing had a flat steel spring coming back to the axis of rotation and serving as the joint. The spring was held in tension by the boom; the only friction occurred internally within the flat spring. This type of joint has been widely used ever since.
A complete summary and discussion of the instruments invented in Japan prior to 1883 was given by Ewing (1883a). At that time, unfortunately, the period of an instrument was not considered too important, except that it was usually intended to be greater then the period of the waves the seismograph was to record. Ewing writes, "When registering on a continuously moving plate, a well-constructed and well-adjusted seismograph should make one complete oscillation in about 5 seconds, and its decrement of amplitude should not exceed about 1 mm. per oscillation" (Ewing, 1883a, p. 78). Although Ewing's damping specification is vague, it suggests that an undamped system was considered most desireable. In fact, we have seen that Perry and Ayrton suggested the use of viscous damping, from theoretical considerations. Gray (1881) and Milne (1881a) successfully used pendulum seismometers which were intentionally heavily damped with solid friction. Ewing (1883a) pointed out that the use of solid friction would mean that there would be a minimum recordable acceleration, below which earthquakes would not record at all.
Figure 13. The Ewing duplex-pendulum seismometer (after Ewing, 1883a). The bob, B1 is connected at A to the bob B2 of the inverted pendulum by means of a "ball and tube joint". The indicator G is attached to the bob of the inverted pendulum.
The use of viscous damping does not seem to have been attempted by the British in Japan. In fields other than seismology, viscous damping had been used to suppress high frequency tremors of pendulums so that the longer period motions could be studied (Rood, 1875; Darwin, 1882). In particular, the attempts of the Darwins (1882) to measure, with a damped pendulum apparatus, the lunar disturbance of gravity, were well known to the British in Japan. But viscous damping was not introduced into seismology until much later.
With the seismometers of Gray and Ewing available, seismologists in Japan were able to make important advances in the understanding of earthquake motion. John Milne, especially, made extensive use of the new instruments.
Figure 14. Ewing's modification of Gray's "conical-pendulum" seismometer (after Ewing, 1883a). The pendulum rotates about P. The member A is forked so that it does not touch P. From the left end of A a flat steel spring comes back to P.
Milne and his colleagues conducted experiments on the propagation of elastic waves from artificial sources (Milne and Gray, 1883; Milne, 1885b). The most revealing records are those of dynamite explosions recorded on moving glass plates by Ewing horizontal-pendulum seismometers and Gray-Ewing vertical seismometers (Figure 15). The moving plates were started into motion shortly before an explosion. Time signals were put on the plates. The records showed the normal motion outracing the transverse, and a slight vertical motion outracing them both. The apparent velocity of the wave of vertical motion was 500 feet per second. The maximum frequency of a disturbance was found to decrease with distance. In evaluating these observations today, it must be understood that the static magnification of the instrument was very low (four or six); early arrivals may not have been recorded. Also, the stations were close to the source (the furthest was 400 feet away), so that inaccurate timing would grossly affect transit velocities. Milne clearly believed that the normal and transverse waves were compressional and distortional body waves, respectively. He thought the vertical displacement was due to "free surface waves" (Milne, 1885b, p. 81). [A number of investigators had written of seismic "surface waves" before Rayleigh (1885) demonstrated mathematically the possibility of surface waves on an elastic medium. The idea of surface waves must have been suggested to earlier scientists by the observation of "waves" on the surface of the ground in an earthquake. J. Le Conte (1882) spoke of "surface waves", but meant the surface projection of body waves. Mallet (1862b, vol. 2, p. 300) envisioned true surface waves, set up by a compressional wave impinging on the surface above the focus of the earthquake, and propagating with velocities which were probably greater than the compressional wave velocity. Mallet's waves were transverse in a vertical plane. It seems likely that Milne thought he was observing the surface waves hypothesized by Mallet.]
Milne placed a number of similar seismometers at different locations to test the effect of topography and geology on earthquake motion (Ewing, 1882b, Milne, 1886a, 1887a). It had long been observed that damage to structures seemed to depend on the "ground" on which they were built (see, for example, Dolomieu (1784) on the Calabrian earthquakes of 1783). Milne found that the amplitude of seismic waves recorded on similar seismometers also depended on the nature of the ground in their immediate vicinities. In "moderately strong" earthquakes, the recorded amplitudes were generally "very much greater" on "soft ground" than on "hard ground" (Milne, 1887a, p. 30-31). For "very small" earthquakes, this was not true. "Soft ground" in Milne's experiment was marshy land where the water-table was a few feet from the surface. "Hard ground" was higher land where the water-table was at least ten feet from the surface. The recording sites used by Milne were separated by several hundred to a thousand feet horizontally, with a maximum difference in elevation of twenty feet. It is worth noting that all the records were started into motion simultaneously by a seismoscope, so that the seismographs should have recorded the same phases. On the other hand, none of the instruments recorded the earliest phases, and there is no way of knowing when in an earthquake they did start recording. In one of the early experiments of this sort, Milne tested the similarity of the seismographs he used by placing them side by side and recording with them simultaneously. He observed that "they practically gave similar diagrams" (Milne, 1884b).
Figure 15. Elastic waves from dynamite explosions (reproduced from Trans. Seis. Soc. Japan, . The distance of each station from an explosion of three pounds of dynamite is given. Time marks are put on every half second. The experiments were carried out in 1881.
Milne performed a similar experiment in different types of building construction and on different floors of the same building (Milne, 1888d). He compared the motion in a brick two-story house and a wood two-story house, situated sixty feet apart on "similar ground". Seismographs were placed on the two floors "vertically above each other and as nearly as possible in similar positions" (Milne, 1888d, p. 67). Milne found that the wood house moved more than the brick hous