  Artificially produced tremors—Observations of Kater, Denman, Airy, Palmer, Paul—Natural tremors—Observations of ZÖllner, M. d’Abbadie, G. H. and H. Darwin—Experiments in Japan—With seismoscopes, microphones, pendulums—Work in Italy—Bertelli, Count Malvasia, M. S. di Rossi—Instruments employed in Italy—Tromometers, microseismographs, microphones—Results obtained in Italy—in Japan—Cause of microseismic motion. During the past few years considerable attention has been drawn towards the study of small vibratory motions of the ground, which to the unaided senses are usually passed by without recognition. These motions are called earth tremors. Their discovery appears to have been due to accident, and not to the results of inductive reasoning. No sooner had philosophers contrived astronomical and other instruments for the purpose of making refined measurements and observations than they at once discovered that they had an enemy to contend against in the form of microscopic earthquakes. Artificially produced tremors.—Artificial disturbances of this description exist in all our towns, and near a railway line they are perceptible with every passing train. Those who have used microscopes of high power will readily appreciate how small a disturbance of the ground is visible in the apparent movement of the object under examination. Captain Kater found that he could not perform his pendulum experiments in London on account of the vibrations produced by the rolling of carriages. Captain Denman, who made some observations on artificially produced tremors, found that a goods train produced an effect 1,100 feet distant in marshy ground over sandstone. Vertically, however, above a tunnel through the sandstone, the effects only extended 100 feet. A remarkable example of the trouble which artificially produced earth vibrations have occasioned those who make astronomical observations occurred some twenty years ago at the Greenwich Observatory. When determining the collimation error of the transit circle by means of the reflexion of a star in a tray of mercury, it was found that on certain nights the surface of the mercury was in such a state of trembling that the observers were unable to complete their observations until long after midnight. After obtaining a series of dates on which these disturbances occurred, it was found that they coincided with public and bank holidays, on which days crowds of the poorer classes of London flocked to Greenwich Park, and there amused themselves with running and rolling down the hill on which the observatory is situated. On these occasions it was found that the disturbances in the mercury were such that observations could not be made until two or three hours after the crowds had been turned out of the neighbouring park.[138] To obviate this difficulty Sir George Airy suspended his dish of mercury in a system of india-rubber bands, and in this way succeeded in eating the intruders up. Lieutenant-Colonel H. S. Palmer, R.E., when engaged with the transit of Venus expedition in New Zealand, in 1874, was troubled with vibrations produced from a Before the United States Naval Observatory was established at Washington, Professor H. M. Paul was deputed to make a tremor survey to discover stable ground. The results of these experiments were exceedingly interesting. By watching the reflected image of a star in a dish of mercury a passing train would be noticed at the distance of a mile. Its approach could be detected by the trembling of the image before its coming could be heard. At one point of observation the disturbance appeared to be cut off by a ravine. The strata was gravel and clay.[140] These few examples of artificially produced tremors, to which many more might be added, have been given because they teach us something respecting their nature. Hitherto earth tremors have only been regarded as intruders, which it was necessary to escape from or destroy. From what has been said they appear to be a superficial disturbance which is propagated to an enormous distance. This distance appears to depend upon the propagating medium, upon the intensity of the initial disturbance, and upon its duration. In the observation of these artificial disturbances, which are accessible to every one, and which hitherto have been so neglected, we have undoubtedly a fruitful source of study. Natural tremors.—Next let us turn to those microscopical disturbances of our soil which are due to natural Some of the more definite observations which have been made upon earth tremors were those made in connection with experiments on the deviation of the vertical due to the attractive influence of the moon and sun. Professor ZÖllner, who invented the horizontal pendulum which he used in the attempt to measure the change in level due to lunar and solar attraction, found his instruments so sensitive that the readings were always changing. The most interesting observations which were made upon small disturbances of the soil were those of M. d’Abbadie, who carried on his experiments at Abbadia, in Subernoa, near Hendaye, 400 mÈtres distant from the Atlantic, and 62 mÈtres above sea level. The soil was a loamy rock. Here M. d’Abbadie constructed a concrete cone 8 mÈtres in height, which was pierced down the centre by a vertical hole or well, which was continued two mÈtres below the cone into the solid rock. At the bottom of this hole or well a pool of mercury was formed which reflected the image of cross wires placed at the top of the hole. These cross wires and their reflection were observed by means of a microscope. The observations consisted in noting the displacement and azimuth of the reflected image relatively to the real image of the wires. After allowing this structure five years to settle, M. d’Abbadie commenced his observations. To find the surface of the mercury tranquil was a rare occurrence. Sometimes the mercury appeared to be in violent motion, although both the air and neighbouring sea were perfectly calm. At times the reflected image would disappear as if the mercury had been disturbed by a microscopic earthquake. The relative positions of the images were in part governed by the state of the tide. Altogether the movements were so strange that M. d’Abbadie did not venture any speculations as to their cause, but he remarks that the cause of the changes he observed were sometimes neither astronomical nor thermometrical. These observations, the principal object of which was to determine changes in level rather than earth vibrations, were carried on between the years 1868 and 1872.[141] Observations at Cambridge.—Another instructive set of observations were those which were made in the years 1880–1882, by George and Horace Darwin, in the Cavendish Laboratory, at Cambridge. The main object in these experiments was to determine the disturbing influence of gravity produced by lunar attraction. The result which was obtained, however, showed that the soil at Cambridge was in such an incessant state of vibration that whatever pull the moon may have exerted upon the instrument which was employed was masked by the magnitude of the effects produced by the earth tremors, and the experiments had, in consequence, to be abandoned. The principle of this instrument was similar to one devised by Sir William Thomson, and put up by him in his laboratory at Glasgow. As erected by the brothers Darwin, at Cambridge, it was briefly as follows: A pendulum, which was a massive cylinder of pure copper, was hung by a copper wire, about four feet long, inside a hollow cylindrical tube rising from a stone support. A small mirror was then hung by two silk fibres, one of which was fastened to the bob and the other to the stone basement. A ray of light sent from a lamp on to the mirror was reflected to a scale seven feet distant, and by this magnification any motion of the bob relatively to As a result of observations like these, the world had gradually forced upon it the fact that the ground on which we live is probably everywhere in what is practically an incessant state of vibration. This led those who were interested in the study of earth movements to establish special apparatus for the purpose of recording these motions with the hope of eventually discovering the laws by which they were governed. Experiments in Japan.—The simpler forms of apparatus which have been used in Japan may be described as delicate forms of seismoscopes, which, in addition to recording earth tremors, also record the occurrence of small earthquakes. A simple contrivance which may be used for the purpose of recording small earthquakes can be made with a small compass needle. If a light, small sensitive compass needle be placed on With crude apparatus like this a large number of small earth disturbances have been recorded. Another form of apparatus, employed in Japan, has been a delicately constructed circuit closer. The motions of this instrument were recorded by causing an electro-magnet to deflect a pencil which was tracing a circle on a revolving dial. The revolving dial was a disc of wood covered with paper fixed to the hour-hand axle of a common clock. A third form of apparatus used in Japan consisted of a small piece of sheet lead about the size of a threepenny piece suspended by a short loop from a rigid support. Projecting from the lead a fine wire, about two inches in length, passed freely through a hole in a metallic plate. By the slightest motion of the support the small pendulum of lead was set into a state of tremor and caused its pointer to come in contact with one or other side of the hole in the metal plate and thus to close an electric circuit. A more refined kind of apparatus which has been employed in Japan was similar to that used by the Darwins at Cambridge. This was so arranged that any deflection of the mirror was permanent until the instrument was reset, and in this way the maximum disturbance which had taken place between each observation was recorded. In addition to these and other contrivances, experiments were made with microphones. The microphones used were small doubly pointed pencils of carbon about three centimetres long, saturated with mercury, and supported vertically in pivot holes bored in other pieces of carbon, which were the terminals of an electric circuit. These microphones were screwed down on the top of stakes driven deeply into the ground. They were covered with a glass shade thickly greased at its base. The stakes were in the ground at the bottom of a small pit—about two feet square and two feet deep—which was lined with a box. The box was covered with a lid, and earth to the depth of nine inches or one foot. One of these pits was in the middle of a lawn in the front of my house, and the other was at the foot of a hill at the back of the house. The wires from the microphone passed through the side of the box into a bamboo tube and thence up to my dining-room and bed-room. In one of the circuits there were three Daniell’s cells, a telephone, and a small galvanometer. I used the galvanometer because I found that when there was sufficient motion in the microphone to produce a sound in the telephone a motion in the needle of a galvanometer was produced. If in any case motion took place in the magnetic needle during my absence, it was held deflected by a small piece of iron with which it was brought into contact by the movement. The sensitiveness of the arrangement may be judged of from the fact that if a person walked on the grass within six feet of the microphone, each step caused a creak in the telephone, and the needle of the galvanometer was caused to swing and come in contact with the iron. Dogs running on the grass had no effect. A small stone one or two inches in diameter thrown from the The nature of the records I received from this contrivance may be judged of from the following extract from my papers.
Here I went out, took away the covering, and examined the microphone. Nothing wrong was to be observed. All that I saw was one small ant. I do not think that this could have caused the disturbance, because it could not get near the instrument. On the succeeding nights I experienced similar disturbances, and it seemed as if they might possibly have been the prelude to several small shocks which occurred about this time (November 15, 16, and 17). On November 17, at 8 a.m., the needle was found in contact, and again at 5 p.m., and at 6 p.m. the shock of a small earthquake was felt which caused a rattling sound in the telephone for about one minute after the motion had appeared to cease. The needle swung considerably, but did not come in contact. The great objection to these observations is that it is possible that the movements and sounds which I have recorded might, with the exception of one case when the shaking was actually felt, possibly have been produced by causes other than that of the movement of the ground. To determine this I subsequently put up two distinct sets The greatest objection to observations of this sort is that the sensibility of the instrument is not constant. After a current has been running for several days it is no longer sensible to slight shocks, it appears as if its resistance had been increased. To overcome this it is necessary to resharpen the carbon points and bore out the pivot holes every three or four days. Farther, the battery varies. This might to some extent be overcome by using a battery with large plates. These two causes tend to reduce the sensitiveness of the galvanometer-like recorder—the deflection of the needle gradually becoming less and less, and therefore day by day needing a greater swing to bring it into contact with the iron. For reasons such as these this instrument, to be used successfully, appears to require considerable attention. Another form of microphone employed by the author consisted of an aluminium wire standing vertically on a metallic plate, its upper end passing loosely through a hole in an aluminium wire standard. The upper end of the vertical wire was loaded with lead. This contrivance possesses all the sensitiveness of an ordinary microphone, whilst, if it receives a sudden impulse, there is a sudden break in the current, and the vertical wire is thrown from one side to the other of the hole in the standard. After many months of tiresome observation with instruments of this description, and after eliminating all motions which might have been produced by accidental causes, the general result obtained showed that in Tokio Work in Italy.—The most satisfactory observations which have been made upon microseismic disturbances are those which have been made during the last ten years in Italy. The father of systematical microseismical research appears to have been Father Timoteo Bertelli, of Florence. In 1870 Father Bertelli suspended a pendulum in a cellar, and observed it with a microscope. As the result of his observations it was announced that he had perceived the earthquakes which shook Romagna, although to the ordinary observer in Florence these shakings had not been perceptible. In 1873 Bertelli, by means of microscopes fixed in several azimuths, made 5,500 observations on free pendulums. He also made observations on reflections from the surface of mercury.[143] One result of these observations was to show that microseismic motions increased with a fall of the barometer. Similar observations were made at Bologna by M. le Conte Malvasia, and also by M. S. di Rossi, near Rome. On January 14, 15, and February 25 these three observers at their respective stations simultaneously observed great disturbances. Similar investigations were made at Nice by M. le Baron Prost. Although doubt was cast upon Bertelli’s observations they appear to have been the origin of a series of microseismical observations, a distinguished leader in which is Professor Rossi, who, in 1874, found that large earthquakes were almost always preceded or accompanied with microseismical Many of Professor Rossi’s observations were made in the grotto of Roca de Papa, 700 mÈtres high and eighteen mÈtres under the soil. Here over 6,000 observations were made by means of microscopes, on pendulums of different lengths, suspended in tubes cut in the solid rock. Instruments employed in Italy.—It is impossible to describe in detail the various forms of apparatus which have been used by the Italian investigators. A description of one or two of the more important instruments may not, however, be out of place, inasmuch as they will assist the reader to understand the manner in which the various results respecting the laws governing microseismic movements have been arrived at. The most important of these instruments is the Normal Tromometer of Bertelli and Rossi. This consists of a pendulum 1½ metres long, carrying, by means of a very fine wire, a weight of 100 grammes. To the base of the bob a vertical stile is attached, and the whole is enclosed in a tube terminated, at its base, by a glass prism of such a form that when looked through horizontally the motion of the stile can be seen in all azimuths. In front of this prism a microscope is placed. Inside the microscope there is a micromatic scale, so arranged that it can be turned to coincide with the apparent direction of oscillation of the point of the stile. In this way not only can the amplitude of the motion of the stile be measured, but also its azimuth. The extent of vertical motion is measured by the up and down motion of the stile due to the elasticity of the supporting Fig. 37.—Normal Tromometer. b, bob of pendulum; p, prism; m, microscope; s, rim of scale. Another apparatus is the Microseismograph of Professor Rossi. Here we have an arrangement which gives automatic records of slight motions. It consists of four pendulums, each about three feet long, suspended so that they form the corners of a square platform. In the centre of this platform a fifth, but rather longer, pendulum is suspended. The four pendulums are each connected just above their bobs to the central pendulum with loose silk threads. Fixed to the centre of each of these threads, and held vertically by a light spring, is a needle, so adjusted that each thread is depressed to form an obtuse angle of about 155°. These needles form the terminals of an electric circuit, the other termination of which is a small cup of mercury placed just below the lower end of the needle. By a horizontal swing of one of the pendulums this arrangement causes the needle to move vertically, but with a slightly multiplied amplitude. By this motion the needle comes in contact with the mercury, and an electro-magnet with a lever and pencil is caused to make a mark on a band of paper moved by clockwork. The Lastly, we have Professor Rossi’s Microphone. This consists of a metallic swing arranged like the beam of a balance. By means of a movable weight at one end of the beam this is so adjusted that it falls down until it comes in contact with a metallic stop. This can be so adjusted that a slight tap will cause the beam to slightly jump from the stop. The beam and the stop form two poles of an electric circuit, in which there is a telephone. The slightest motion in a vertical direction causes a fluctuation in the current passing between the stop and the beam, and a consequent noise is heard in the telephone. With instruments analogous to these, observations have been made by various observers in all portions of Italy, extending over a period of ten years. Every precaution appears to have been taken to avoid accidental disturbances, and the experiments have been repeated in a variety of forms. Results obtained in Italy.—The results which from time to time have been announced are of the greatest interest to those who study the physics of the earth’s crust, and they appear to be leading to the establishment of laws of scientific value. It would seem that the soil of Italy is in incessant movement, there being periods of excessive activity usually lasting about ten days. Such periods are called seismic storms. These storms are separated by periods of relative calm. These storms have their greater regularity in winter, and sharp maximums are to be observed in spring and autumn. In the midst of such a period or At the commencement of a storm the motions are usually small, and one storm, lasting two or three days, may be joined to another storm. In such a case the action may be a local one. It has been observed that a barometrical depression tended to bring a storm to a maximum, whilst an increase of pressure would cause it to disappear. Sometimes these actions are purely local, but at other times they may affect a considerable tract of land. If a number of pendulums of different length are observed at the same place, there is a general similarity in their movements, but it is also evident that the free period of the pendulum more or less disturbs the character of the record. The greatest amplitude of motion in a set of pendulums is not reached simultaneously by all the pendulums, and at every disturbance the movement of one will predominate. From this Rossi argues that the character of the microseismical motions is not constant. Bertelli observed that the direction of oscillation of the pendulums is different at different places, but each place will have its particular direction dependent upon the direction of valleys and chains of mountains in the neighbourhood. Rossi shows that the directions of movement are perpendicular to the direction of lines of faults, the lips of these fractures rising and falling, and producing two sets of waves, one set parallel to the line of fracture, Fig. 38. The disturbances, as recorded at different towns, are not always strictly synchronous, but succeed each other at short intervals. If, however, we take monthly curves of the disturbances as recorded at different towns in Italy, we see that these are similar in character. The maximum of disturbance occurs about the winter solstice, and the minimum about the summer solstice, and in this respect they exhibit a perfect accordance with the curves drawn by Mallet to show the periodicity of earthquakes. The accompanying curves taken from one of Bertelli’s original memoirs not only show this general result but also show At Florence, before a period of earthquakes there is an increase in the amplitude and frequency of vertical movements. These vertical movements do not appear to coincide with the barometrical disturbances, but they appear to be connected with the seismic disturbances. They are usually accompanied with noises in the telephone, but as the microphone is so constructed as to be more sensitive to vertical motion than to horizontal motion, this is to be expected. This vertical motion would appear to be a local action, inasmuch as the accompanying motions of an earthquake which originates at a distance are horizontal. Storms of microseismical motions appear to travel from point to point. Sometimes a local earthquake is not noticed in the tromometer, whilst one which occurred at a distance, although it may be small, is distinctly observed. To explain this, Bertelli suggests the existence of nodes. Similar conclusions were arrived at by Rossi when experimenting on different portions of the sides of Vesuvius. Galli noticed an augmentation in microseismic activity when the sun and moon are near the meridian. Grablovitz found from Bertelli’s observations a maximum two or three days before the syzygies, and a minimum three days after these periods. He also found that the principal large disturbances occurred in the middle of periods separating the quadrature from the syzygies, the apogee from perigee, and the lunistigi period from the nodes, whilst the smallest disturbances happened in the middle of periods opposed to these. P. C. Melzi says that the curves of microseismical motions, earthquakes, lunar and solar motions, show a concordance with each other. With the microphone Rossi hears sounds which he describes as roarings, explosions, occurring isolated or in volleys, metallic and bell-like sounds, ticking, &c., which, he says, revealed natural telluric phenomena. Sometimes these have been intolerably loud. At Vesuvius the vertical shocks corresponded with a sound like volleys of musketry, whilst the undulatory shocks gave the roaring. Some of these sounds could be imitated artificially by rubbing together the conducting wires in the same manner in which the rocks must rub against each other in an earthquake. Other sounds were imitated by placing the microphone on a vessel of boiling water, or by putting it on a marble slab and scratching and tapping the under side of it. These, then, are some of the more important results which have been arrived at by the study of microseismic motions. One point which seems worthy of attention is that they appear to be more law-abiding than their more violent relations, the earthquakes, and as phenomena in which natural laws are to be traced they are certainly deserving our attention. As to whether they will ever become the means of forewarning ourselves against earthquakes is yet problematical. Their systematic study, however, will enable us to trace the progress of a microseismic storm from point to point, and it is not impossible that we may yet be enabled to foretell where the storm may reach its climax as an earthquake. These, I believe, are the views of Professor di Rossi, who is at the present time engaged in the establishment of a system of microseismic observations throughout Italy. Before the earthquake of San Remo (Dec. 6, 1874) Rossi’s tromometer was in a state of agitation, and similar disturbances were observed at Livorno, Florence, and Bologna. Since February 1883 I have observed a tromometer in Japan, and such results as have been obtained accord with results obtained in Italy. The increase in microseismical activity with a fall of the barometer is very marked. Other peculiarities in the behaviour of the instrument will be referred to under ‘Earth Pulsations.’ Cause of microseismic movements.—As to the cause of tromometric movements, we have a field for speculation. Possibly they may be due to slight vibratory motions produced in the soil by the bending and crackling of rocks produced by their rise upon the relief of atmospheric pressure. If this were so we should expect similar movements to be produced at the time of an increase of pressure. Rossi suggests that they may be the result of an increased escape of vapour from the molten materials beneath the crust of the earth, consequent upon a relief of pressure. The similarity of some of the sounds which are heard with the microphone to those produced by boiling water are suggestive of this, and Rossi quotes instances when underground noises like those which we should expect to hear from a boiling fluid have been heard before earthquakes without the aid of microphones. One instance was that of Viduari, a prisoner in Lima, who, two days before the shock of 1824, repeatedly predicted the same in consequence of the noises he heard. A possible cause of disturbances of this order may be small but sudden fluctuations in barometric pressure, which are visible during a storm. During a small typhoon on September 15, 1881, when in the Kurile Islands, I observed that the needle of an aneroid worked back and forth with a period of from one to three seconds. This continued for several hours. With every gust of wind the needle suddenly rose and then immediately fell. At times it trembled. These movements were observed in the open An inspection of the following few observations taken from my note-book for the same typhoon will suggest that even the large and slower variations are capable of producing tremulous motions.
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