Influence of Instruments: From Descriptive Earthquake Studies to Seismology as an Exact Science
In a publication praised at the Kaiser-Wilhelms-Universität Strassbourg that contains a 'compilation, description and critical evaluation of the most important seismometers with special reference to their practical usability', including more than 200 devices and thus being the most detailed presentation of earthquake instruments known until the end of the 19th century, its author, R. EHLERT, wrote:

"During an earthquake the main purpose of seismometers is to monitor exactly the movement of the ground where they are mounted. As long as it was not possible to monitor these movements, we could not establish correct principles for the construction of earthquake instruments, as they themselves were supposed to help us by their recordings in establishing a theory of seismology."

"Seismometer haben in erster Linie den Zweck, die Bewegung ihres Aufstellortes während eines Erdbebens konform abzubilden. Solange man nun die Art dieser Bewegungen nicht erkannt hatte, war es unmöglich, sogleich richtige Prinzipien für die Konstruktion von Erdbeben-apparaten aufzustellen, da dieselben ja erst durch ihre Aufzeichnungen die Theorie der Seismologie erschließen sollen."[1]

According to this statement there is not only the influence of instruments on the theories of earthquake movements, but also vice versa the influence of theories on the construction of such earthquake instruments. But these 'circular processes', as EHLERT says, between the theory of constructing observation instruments and the theories on the dynamics of earthquakes are just another step in the interaction between theory and observation. For such circular processes, as we have shown already[2], also happen on the merely qualitative pre-instrumental level of observation between the non-instrumental earthquake data and the theories on the causes of earthquakes, in the sense that observations non only determine the theories, but also vice versa that observations are determined by 'anticipating theories'[3].

In this investigation of the theories of the nature and mechanisms of earthquakes in the 19th century we therefore concentrate on those instruments whole principles of construction are determined by the theoretical ideas of the type of ground movements that occur during earthquakes. In a subsequent step we will focus on how the results gained from applying these instruments have influenced the theories of the nature and mechanisms of earthquake movements.

The historical point of departure for this mutual influence and interactive development of earthquake theories and earthquake instruments are the early proposals for constructing such devices, as they were included by MALLET and v. SEEBACH in their earthquake theories.

Immediately after his theoretical pioneering work on the 'dynamics of earthquakes' in 1846 in the Irish Academy of Sciences, MALLET added a brief treatise 'On the Objects, Construction, and Use certain new Instruments for Self-registration of the Passage of Earthquake Shocks' that he extended in a later report, the 4th report to the British Association for the Advance of Science in 1858, to a considerable degree by new ideas about a 'perfect seismometer' and that he also complemented by a brief history of earthquake instruments. This treatise is one of the most important original sources on the development of earthquake instruments and of instrumental exact seismology as such. It was the basis for subsequent historical accounts by MILNE (1886) and EHLERT (1896) that in turn were the basis for the textbook by SIEBERG (1904).

MILNE, who developed a seismograph that was named after him, extended the instrumental seismometry not only by the description of new instruments produced in Japan, but also by adding a contribution to the knowledge on the historical origin of earthquake instruments in China.

According to these sources, the historical development and sequence of the individual inventions and the systematic typology according to theoretical principles of construction shall be presented in the following account. As EHLERT already pointed out, this description will help us in understanding the development of these instruments.[4] This investigation will remain limited to mechanical earthquake instruments as they had been developed until the end of the 19th century and as they have been used in their basic constructions until the second half of the 20th century.

After a long historical development with its origin going back to the first traces of earthquake research in China, when attempts were rather based on intuition than on theory by objectifying by mechanical devices the subjective human perception of different ground movements during earthquakes, three phases can be distinguished in the actual development of earthquake instruments in the 19th century:[5]

  1. The origins of instrumental observation and the origins of seismology as an exact science. During this period, a fundamental distinction was introduced between the processes in the center of the earthquake and the propagation of the earthquake waves. Alternative theories of nature and mechanisms of earthquake waves were developed. The first rather complicated instruments for measuring earthquakes automaticaley in order to confirm the theory - were also developed.
  2. The observation of nearby earthquakes by automatically recording seismometers that already were able to follow the whole process of vibrations during the whole earthquake.
    This was the time when the interaction, mentioned above, between the development of theories on the types of vibrations and the paths of earthquake waves and the development of measuring devices occurred:[6] Progress in the theory of the wave movement of earthquakes led to the construction of new instruments and vice versa.
  3. The registration of remote earthquakes (1890-1900), where the recordings by appropriate precision instruments show more regularities than those of the near earthquakes where confusion arises due to the secondary movements of the epicentre.

This phase began with European observations of Japanese earthquakes, as they were first carried out in Germany by E.v. REBEUR-PASCHWITZ with his horizontal pendulum.

The point of departure for this historical development was in the first place the rough distinction between two classes of eathquake instruments that later led to the differentiation between 'seismoscopes' an 'seismometers'. MALLET called these seismoscopes 'observational seismometers'. These are those earthquake instruments whxose movements have to be recorded by a human observer after each earthquake shock. A different type are the "self-registering" seismometers in the true sense, that possess a device for recording and mapping in different ways the ground movement, were later therefore called 'seismographs' or 'seismometer-graphs'. Each of these two major classes were categorized by MALLET into two further types: a) those that are based on the movement or displacement of fluids; and b) those that depend on the partial movement of solid bodies.[7] Obviously it is difficult to draw an exact line of division between these two classes of earthquake instruments. According to the requirements to the precision or to the registration methods, the first generation of earthquake instruments, in particular the fluid pendula and rolling or overthrown solid bodies were only called seismoscopes, until they were replaced by precision instruments automatic recording.

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