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SKETCH OF THE HISTORY OF MICROSEISMOLOGY J. B. Macelwane, S.J. St. Louis University John Milne, who is known as the father of modern seismology, published a paper [1883- 84] in which he made the following statement. "The father of microseismical research seems to have been Father Timoteo Bertelli of Flor- ence. In 1870 Father Bertelli suspended a pendulum in a cellar which he observed with a telescope ... 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 sur- face of mecury." Bertelli [1875] would seem to deserve the title given him by Milne because he appears to have been the first to undertake systematic studies of microseisms and because his publications moved so wide a circle of in- vestigators to undertake research on micro- seisms and because he gave this name to the phenomenon. Actually the early observations of Bertelli referred to by Milne extended over the three years, 1869 to 1872. By 1874 daily observations were made at five stations in Italy and by 1884 at thirty. Bertelli was, of course, not the first to ob- serve that the surface of the earth is in a state of more or less continuous agitation. Astrono- mers and geodesists using a pool of mercury as a reference level found the surface of the mer- cury rarely quiet. George H. and Horace Dar- win [1881], who had set up elaborate apparatus at Cambridge, England, to observe lunar tides with a magnification of 50,000 times, found such incessant ground vibrations that the ex- periment had to be abandoned. However, they seem to have made no attempt to study the vibrations as such. John Milne in Japan interested himself very early in the observation of microseisms. In a paper read before the Seismological So- ciety of Japan, Milne [1881] described a series of experiments he had made to determine the characteristics of microseisms, including the use, in February, 1880, of rotating mirrors with a magnification of approximately 250 times. He concluded: "From these results it would at first sight appear that the ground in Tokyo is almost constantly in a state of tremor." Two years later in a paper read before the Seismo- logical Society of Japan Milne [1883] described the observation of earth vibrations in Italy, France and England and concluded: "Like ob- servations have been made in Japan and it does not seem improbable that after farther experiments have been carried out we shall be brought to the conclusion that the surface of the whole globe is affected by similar micro- seismical disturbances." Milne noted the suc- cession of intervals of comparative quiet fol- lowed by periods of hours or days of large amplitude disturbance, and he introduced the term "microseismic storm" to describe the lat- ter. He presented a tabulation [Milne 1887] of a long series of observations of the north- south and east-west components of microseisms together with earthquakes, barometric heights, wind velocities and their gradients. He said: "In conclusion, so far as my observations have gone in Japan, it appears that the majority of earth tremors are movements produced by the action of the wind upon the surface of the earth and that these may often be propagated to dis- tant places where wind disturbances have not occurred." In Germany during the years 1892-1894 E. von Rebeur-Paschwitz was engaged in the observation of earth tides by means of hori- zontal pendulums. In the report [1895a] on his observations at Strasbourg a section [1895b] entitled "Die Mikroseismische Bewe- gung" was devoted to his observations on mi- croseisms. These observations were continued at Strasbourg by Ehlert [1898]. With the beginning of the Twentieth Cen- tury, interest in microseisms had become gen- eral. In Japan F. Omori [1901] summed up the results of his observations in these words. "The chief characteristics of these movements, as observed in Tokyo, are the following:— 1. Pulsatory oscillations occur more fre- quently in winter than in summer. 2. Pulsatory oscillations continue gener- ally for several days, there being no dependence of the frequency on the time of day. 3. The average period remains generally constant for several hours, not depending much on the amplitude. 4. The average period varies but little, the least value being 3.4 s. and the greatest value 8.0 a.

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SYMPOSIUM ON MICROSEISMS 5. The direction of motion changes con- stantly, and each horizontal component shows a series of alternations of maximum and mini- mum groups; .... the average period being 6.8 s " Subsequently Omori [1903] wrote: "It thus seems probable that the pulsatory oscilla- tions are essentially composed of two series of vibrations, whose periods are respectively about 4 sec. and 8 sec.; large pulsatory move- ments which are caused by very deep cyclones having generally the 8 seconds period." Ten years later Omori [1913] reported a comparison of the microseisms recorded at Tokyo with those recorded at Hitotsubashi about two kilometers distant in which he found it impossible to identify individual vibrations on the two sets of records. This negative re- sult, which may have been due to instrumental deficiencies probably led Honda [ ] and other later Japanese investigators to adopt the view that microseisms are stationary waves set up at the locality of the observing station. At the meetings of the International Seis- mological conferences and later of the Interna- tional Seismological Association, microseisms were of interest from the first and a standing committee for the study of microseisms was soon appointed. It was at the second Inter- national conference held at Strasbourg in 1903 that Wiechert [1905] proposed his well-known surf theory. Laska [1902] on the other hand correlated maximum amplitudes of micro- seisms with steepest barometric gradients on the basis of the records at Lemberg. Klotz [1909] presented a report at the Zermatt meet- ing of the International Seismological Associa- tion in 1908 in which he said among other things: "5. A well-marked Low sweeping up the Atlantic Coast from Florida to Newfound- land is almost always accompanied by marked microseisms. 6. Microseisms are but slight- ly, if at all, influenced by the movements of Lows across the continent." It must be remembered that in nearly all of these early investigations attention was fo- cussed on the band of microseismic frequencies which lies between one-fourth and one-eighth herz or those of still lower frequency because these microseisms were so prominent on the records of the seismographs then in use. The study of microseisms of higher frequency be- came possible much later with the introduc- tion of more suitable types of instrumentation. Gutenberg [1910] published his doctoral dissertation at Goettingen in which he pre- sented extensive data which he interpreted as supporting Wiechert's surf theory and he cor- related the microseisms of four to ten seconds in Germany with surf on the southern part of the west coast of Norway. This conclusion he supported vigorously through the succeeding years [Gutenberg 1912, 1921, 1924, 1927, 1928, 1931, 1936]. More recently he has modified and broadened his views in accord with his ex- cellent later researches. In addition to the surf theory, three other main theories or groups of theories have been advanced to explain microseisms. These are first, theories of local origin, meteorological or geological, at or near the recording station, secondly, theories of thermometric or baro- metric gradients travelling over continental areas; and thirdly, theories connected with storms or storm waves at sea. The last named theory casually proposed by Bertelli [1878] and by Omori [1903] and formally reported by Klotz [1909] as a result of his observations at Ottawa, and specifically formulated and defended by Gherzi [1923, 1924, 1926a, 1926b, 1928, 1930, 1937], by Ban- erji [1929, 1930, 1935] and by Zanpn [1936, 1938] in the nineteen twenties and thirties, has come to occupy the center of the modern micro- seismic stage. Most of these studies were made by tem- poral correlation, as were a number of more recent investigations. At the second meeting of the Eastern Section of the Seismological So- ciety of America held in Ottawa, Canada, a committee was appointed to correlate and map microseismic amplitudes recorded at all seismo- graphic stations in the United States and Can- ada. A more ambitious program of correla- tion has been inaugurated by the Association of Seismology of the International Geodetic and Geophysical Union in 1952 involving simul- taneous observations of microseisms in the whole world. However, many seismologists have felt that temporal correlation of amplitudes and periods from place to place is not sufficient to distinguish between possible sources and have sought to determine the bearing of the origin through measurements of the direction of prop- agation of the microseismic waves. Some have attempted to do this by vector methods, using the separately recorded components on the as- sumption that microseisms are Rayleigh waves. Others beginning with Omori [1913] and Hecker [1915] attempted to find the direction independently of any assumption by means of the time interval between arrivals at closely spaced recording stations but failed because of inadequate instrumentation. Shaw [1922] in the years 1918 to 1922 obtained some evidence by this method that microseisms arrived at West Bromwich, England, from the northwest. Krug [1937] at Goettingen in 1936-1937 used three individually timed portable seismographs at the corners of an isoceles triangle with lim- ited success. But the following year 1937 Trommsdorff [1939] and Ramirez [1940], in- dependently using the simultaneously timed tri- partite station method, found that the bear-

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SKETCH OF THE HISTORY OF MICROSEISMOLOGY ings determined from arrival times indicated the position of a low pressure area over the sea. The Ramirez method was applied to the detection and tracking of hurricanes and ty- phoons by the United States Naval Aerological Service under Gilmore in 1944 [Gilmore 1946] and the following years. Anomalies in the in- dicated bearings, laid in part to refraction, have prevented the operational use of the meth- od and have inspired critical investigations by Donn, VanStraaten, Kammer and Dinger, and others which still leaves the problem of the generating mechanism unsolved. Imbo [1931] then director of the seismological station at Catania, Sicily, published in 1930 a comparison of the periods of microseisms and of waves in the Mediterranean Sea and showed that the sea wave periods were twice as long. A simi- lar result was obtained in 1947 by Deacon [1947] and in 1950 by Darbyshire [1950]. This relationship inspired a theoretical investigation by Longuet-Higgins [1950] which indicated that the second order terms in the equations of the pressure field produced by standing waves at sea integrated over a sufficiently large area could account for microseisms of half the period of the standing waves. An example of the complexities involved in the problem of determining the cause or causes of microseisms of the types under discussion was brought out in the discussions at the "Study Week on Microseisms" which met at Rome in November, 1951, under the auspices of the Pontifical Academy of Sciences. While the "group" or "beat" form of the storm micro- seisms was a characteristic taken for granted by the participants from the Western Atlantic and Western Pacific stations it seemed to be less familiar to the European participants at their stations. Caloi in Rome presented argu- ments and observational data in favor of ma- rine barometric gradients as a source. Coming now to other bands of frequencies, research is still in its infancy on microseisms of two to three seconds period yet certain facts have been ascertained concerning them. In the case of "group" microseisms of four to eight seconds period, Klotz, Gherzi, Ramirez, Donn and others observed that the amplitudes rise rapidly and take on their characteristic form when a low pressure area leaves the continent and enters the ocean and that the amplitudes fall rapidly and the waves lose their regular form when the storm leaves the water and en- ters the land. This seems not to be the case with the microseisms of two to three seconds period. At Corpus Christi, Texas, they were found by Jennemann to arrive from the north at the tripartite station operated there by the United States Navy Aerological Service, and hence must have originated and been propa- gated on the North American continent. Father Lynch of Fordham, on contract with the Office of Naval Research, found that in southern New York state they arrive from the west and in North Carolina from the northwest. Still less is known about the microseisms of frequency two to three which are widely ob- served in the records of open time-scale, short period seismographs. Research on those mi- croseisms is in progress at Saint Louis on con- tract with the Office of Naval Research but has not progressed far enough to warrant any con- clusions. Geophysical prospectors are familiar with microseisms of still higher frequency which form unwelcome background noise in their operations. But, as far as the writer is aware, no systematic study has been published con- cerning them. REFERENCES BANERJI, S. K., Microseisms associated with storms in in the Indian seas, Nature, v. 123, pp. 163-164, 1929. BANERJI, S. K., Microseisms associated with disturbed weather in the Indian seas, Phil. Trans. Roy. Soc., London, v. 229, pp. 287-328, 1930. BANERJI, S. K., Theory of microseisms, Proc. Ind. Acad. Sci., v. 1, pp. 727-753, 1935. BERTELLI, I., Comptes Rendus, 1875, January to June, p. 685, Atti. Accad. Pontif. Nuovi Lincei, Anno XXI, sess. 2a, 1878. DARBYSHIRE, J., Identification of microseismic activity with sea waves, Proc. Roy. Soc., London, A v. 202, pp. 439-448, 1950. DARWIN, G. H., and HORACE DARWIN, Reports on the British Assoc., 1881. DEACON, G. E. R., Relation between sea-waves and mic- roseisms, Nature, v. 162, pp. 419-421, 1947. EHLERT, R., Horizontalpendelbeobachtungen im Meridi- an zu Strassburg, Gerlands Beitr. z. Geoph., v. 3, pp. 193-201, 1898. GHERZI, E., Etude sur les microseismes, Notes de Seis- nwlogie, Zi-ka-wei, No. 5, pp. 1-19, 1923, No. 8, pp. 1-12, 1926a. GHERZI, E., Microseismes et deferlement des vagues sur les cotes, Zeit. f. Geoph., v. 1, pp 163-164, 1924, v. 2, p. 159, 1926b. GHERZI, E., Le probleme des microseismes a groupes, Zeit f. Geoph., v. 4, pp. 147-151, 1928. GHERZI, E., Microseisms associated with storms, Ger- lands Beitr. z. Geoph., v. 25, pp. 145-147, 1930. GHERZI, E., Recherches sur la periode de U a 6 secondes de microseismes a groupes, Notes de Seismologie, No. 12, pp. 1-8, 1937. GILMORE, M. H., Microseisms and ocean storms, Bull. Seism. Soc. Am., v. 36, pp. 89-119, 1946.

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SYMPOSIUM ON MICROSEISMS GUTENBURG, B., Die Seismixchc Bodenruhe, Inaugural- Dissertation. Goettingen, 1911. GUTENBURG, B., Die seismische Bodenruhe, Gerlands Beitr. z. Geoph., v. 11, pp. 314-353, 1912. GUTENBURG, B., Untersuchungen uber die Bodenruhe mit perioden von JfS-lOs, in Europa, Veroeffentl. des Zentralbur. der Int. Seism. Assoc., 1914, pp. 1-106, 1921. GUTENBURG, B., Die seismische Bodenruhe in Zi-ka-wei, Zeit. f. Geophys., v. 1, pp. 69-70, 1924. GUTENBURG, B., Die Bodenruhe durch Brandung, Zeit. f. Geophys., v. 3, pp. 328-329, 1927. GUTENBURG, B., Bodenruhe durch Brandung und durch Frost, Zeit. f. Geoph., v. 4, pp. 246-250, 1928. GUTENBURG, B., Microseisms in North America, Bull. Seism. Soc. Amer., v. 21, pp. 1-24, 1931. GUTENBURG, B., On Microseisms, Bull. Seism. Soc. Amer., v. 26, pp. 111-117, 1936. HECKER, O., Versuche zur Bestimmung der Fortpftant- zungsgeochwindigkeit der Boden, bewegung bei der mikr. unruhe, Gerlands Beitr. z. Geoph., v. 14, pp. 28-33, 1915. * HONDA, H., Geophys. Mag., v. 8, pp. 177-181. IMBO, G., Agitazione del mare e microsismika a Cantan- ia, Boll. Soc. Sismol. Ital., v. 29, pp. 165-169, 1931. KLOTZ, O., On microseisms, C. R. Int. Assoc. Seism. Zermatt, pp. 73-74, 1909. KRUG, H. G., Ausbreitung der naturlichen Bodenun- ruhe nach Aufzeichnungen mit transportablen Hor- izontalscismographens, Zeit. f. Geoph., v. 13, pp. 328-348, 1937. LASKA, W., Ueber pendelanruhe, Verhandl. der 1. In- tern. Seism. Konf. Strassburg 1901, Gerlandf Beitr. 2. Geoph., Ergaenzungsband 1, pp. 209-235, 1902. LONGUET HlGGINS, M. S., A theory of the origin of mic- roseisms, Phi. Trans. Roy. Soc. London, A, v. 257, pp. 1-35, 1950. MlLNE, J., Trans. Seismol. Soc. of Japan, v. Ill, pp. 12-61, 1881. MILNE, J., Earth pulsations, Trans. Seismol. Soc. of Japan, v. VI, pp. 1-12, 1883. MILNE, J., Earth tremors, Trans. Seismol. Soc. of Japan, v. VII, pp. 1-15, 1883-1884. MILNE, J., Earth tremors in Central Japan recorded by an automatic tremor recorder, Trans. Seismol. Soc. of Japan, v. Xi, pp. 1-78, 1887. OMORI, F., Publications of the Earthquake Investiga- tion Committee in Foreign Languages, Tokyo, No. 5, pp. 1-82, 1901. OMORI, P., Publications of the Earthquake Investigation Committee in Foreign Languages, Tokyo, No. 13, p. 84, 1903. OMORI, F., Report on the observation of pulsatory oscil- lations in Japan, Bull, of the Earthquake Invest. Com., v. 5, pp. 109-147, 1913. RAMIREZ, J. E., An experimental investigation of the nature and origin of microseisms at St. Louis, Mo., Bull. Seism. Soc. Am., v. 30, pp. 35-84, 139-178, 1940. VON REBEUR-PASCHWITZ, E., Horizontalpendel-beobach- tungen auf der K. Universitats-stemwzrte zu Strassburg, Gerlands Beitr. z. Geoph., v. 2, pp. 211-536, 1895. SHAW, J. J., Communication de M. J. J. Shaw sur les mouvements microseismiques, Comptes-i endus des Seances, Prem. Con/. Rome, I.G.G.U., pp. 52-53, 1922. TROMMSDORFF, FRO., Untersuchen uber die naturliche Bodenunruhe (mikroseismik) mit transportablen Dreikomponentstationen, Zeit. f. Geoph., v. lo, pp. 304-320, 1939. WIECHERT, E., Verhandl. der 2 Intern. Seism. Konf., 1905, Snppl. 2, pp. 41-44, Gerlands Beitr. z. Geoph., Ergaenzungsband 2, pp. 41-42, 1905. ZANNON, F. S., / cicloni dell'atlantico settentrionale i microsismi a Venezia Boll. Soc. Sism. Italiana, v. 34, pp. 35-53, 116-133, 1936. ZANON, F. S., / microsismi registrati in Venezia causati da cicloni sull Atlantico e sull Adriatico, Boll. Soc. Sism. Italiana, v. 35, pp. 219-226, 1938. Discussion B. GUTENBERG California Institute of Technology The paper by Father Macelwane gives an excellent summary. However, the reported statement by European seismologists, that beats are not observed in European micro- seisms, does not correspond to the facts and is probably caused by an incorrect translation of the expression "beats" by scientists from Cen- tral Europe. "Schwebungen" (beats) are dis- cussed in several European and especially Ger- man publications on microseisms (see e.g. the author's Handbuch der Geophysik, vol. 4, p. 282). They are also frequently found in mi- croseisms recorded at Pasadena and other sta- tions near the Pacific coast. There is no evi- dence that beats or groups in the regular type of microseisms with periods of 4-10 seconds are restricted to certain areas. A few words may be added concerning the history of the division of microseisms into dif- ferent types. The first detailed description of such types was given by Hecker [1906]. He distinguished four kinds of microseisms de- pending on whether the period was less than 4, about 7, about 30 seconds, or of the order of 1 minute. The first type is usually caused by local disturbances. Gutenberg [1910, 1912]

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SKETCH OF THE HISTORY OF MICROSEISMOLOGY suggested the division of microseisms into two major groups: a) microseisms caused by local effects and b) by distant sources of energy. Group a) included microseisms from traffic and industry with periods of less than 2 sec- onds, from local wind storms and from local surf. Group b) consisted of microseisms from ocean waves (surf) with periods from 3-10 seconds, of microseisms with periods of about y% minute (from distant wind storms?—this type may be spurious and these "microseisms" may have been caused by air currents in the instrument vault), and of microseisms with periods of 1 or more minutes during periods of frost near the station. Later, additional types were reported [Gutenberg and Andrews 1951, Gutenberg 1951]. It is very important to recognize the type which is recorded in a given instance. Several types from apparent- ly different causes have about the same periods, but differ in their appearance. REFERENCES HECKER, O., Seismometrisehe Beobaehtungen in Pots- dam, Veroff. Preug». Geodat. ln»t., v. 29, 1906. GUTENBURG, B., Ueber seismisehe Bodenunruhe, Phy. Zeit., v. 11, pp. 414-450, 1910. GUTENBURG, B., Die seismische Bodenunruhe, Gerlands Beitr., Geophys., v. 11, pp. 314-361, 1912. GUTENBURG, B., and ANDREWS, F., Bibliography on Mi- croseisms, Mim. Rpt., Seismol. Lab., Pasadena, 1951. GUTENBURG, B., Compendium of Meteorology, pp. 1303- 1311, 1951. Discussion JAMES TINLEY WILSON University of Michigan Father Macelwane has given an excellent summary of the early history of the study of microseisms and of the steps which have led to our present state of knowledge. I do not feel it necessary to comment in detail on Father Macelwane's remarks, rather I would prefer to present some further comments. Most, if not all, of this information is already known to Father Macelwane and to the rest of you but it constitutes more of the data that we must keep in mind when considering the sub- ject. The fact that the study of microseisms is not a closed book is evidenced by this confer- ence. If further evidence be needed, we have but to remember that there is not complete agreement among seismologists as to either the nature or the origin of microseisms, al- though they have been subject to study for some seventy-five years. Certain aspects of the subject are, of course, fairly clear. For example, Omori's observations in 1901 quoted by Father Macelwane would not look too out of place in a modern publication. A seismolo- gist of the present day might study his seismo- grams (and the literature) for some time and not do much better. The problem is such that we cannot afford to neglect any possibilities and must as far as possible consider all of the observations. Mi- croseisms being almost always with us, we usu- ally have more of the latter than we know what to do with. The items I am going to mention come for the most part under the headings of "nature" or "origin." As Father Macelwane has done, I will limit myself primarily to those micro- seisms which have periods in the range of three to twelve seconds. Although a case has been made on both observational and theoretical grounds for mi- croseisms being stationary waves of some sort of "free vibration," the present concensus of opinion seems to be that they are traveling waves. The usual assumption has been that they are surface waves and more specifically of the Rayleigh type. I cannot help but feel that the latter assumption is based almost solely on the fact that they have a vertical component. The suggestion has been made, of course, that they are a mixture of Love and Rayleigh waves. In this connection it might be pointed out that attempts to obtain the direction of approach by comparing the phase of horizontal and vertical components on the assumption that the micro- seisms are Rayleigh waves has not led to as good results as the well known tripartite meth- od. Various attempts have been made to com- pare the observed periods and amplitudes with those expected from Rayleigh waves in certain types of crustal structures. The results have usually been tantalizing but not conclusive. While on the subject of the nature of mi- croseisms, mention might be made of factors also related to origin. As noted before, it has been a common observation that microseisms are larger in the winter than in the summer and, in at least a general way, I think it can be said that they are larger in coastal regions than in the continental interiors. Further, all seismogram borrowing seismologists know that there are certain stations, for example Perth, where the microseisms are a constant nuisance. This variation with season and geography seems to have fathered some of the theories of microseismic origin. Wiechert's surf theory and the oceanic storm theories of Gherzi and

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8 SYMPOSIUM ON MICROSEISMS Banerji might be cases in point. I do not pass judgment on these theories here, but merely wish to indicate that there is a proximity fac- tor in their historical development. . As Father Macelwane has pointed out, there are three general theories for the origin of microseisms (1) local meterological or geo- logical conditions, (2) meterological gradients over continental areas, and (3) meterological conditions at sea. There seems to be almost universal agreement that the source is meteor- ological. Two problems have been considered at length in this connection; first, what meteor- ological conditions supply the energy to the ground, and secondly, how is the energy trans- ferred. The source of the energy is usually sought in "active" meteorological situations such as fronts or low pressure areas. Water bodies are frequently considered as a coupling medium to pass the energy into the ground. As mentioned by Father Macelwane, a long list of seismologists and meteorologists have for- mulated theories or portions of theories, but no one of them seems to explain all of the mi- croseisms all of the time. Some of them fool some of the microseisms some of the time, and one might even say that some of them fool some of the microseisms all of the time. I think a fact that is sometimes forgotten in trying to assess the various theories is that all of them must, of mathematical necessity, deal with rather idealized cases that may fit a given piece of geography fairly well, but will fail rather badly to match in other parts of the world. In this connection, I think it is an historical fact, and so worth mentioning in this historical discussion, that after any seismolo- gist has tended the same station for a few decades he begins to know his own microseisms fairly well and, if he has become interested in them, he is likely to be able to relate his mi- croseisms rather consistently to certain weath- er conditions, but he still may not be able to formulate a theory that will stand up for all the other stations in the world. In studying both the nature and origin of microseisms, seismologists have made consider- able use of large masses of data. Some of this has been done out of necessity in an attempt to extract useful microseism data from seismo- grams run for the routine recording of earth- quakes. In other cases it has been in an at- tempt to obtain correlations between microseis- mic activity and weather conditions. To me, a very interesting transition has taken place in the past ten or fifteen years with more empha- sis now on the study of individual microseismic storms and, in many cases, with the aid of seismographs specifically designed and oper- ated for recording them. To some this might seem like a tree-to-tree examination before we have seen the forest, but having been unable for so long to get a clear picture of the forest, it may well be the proper method. Discussion from the Floor Bath. Dr. Gutenberg emphasized the parallel behavior of microseisms in northern Europe. In a comparison of microseisms at Uppsala, Bergen, and Copenhagen, this result was con- firmed as far as the broad outlines are con- cerned, but there were significant deviations in detail. These could be explained by the hypothesis of an origin along a line source, but were not in accord with the hypothesis of a point source.