NATURE 



337 



THURSDAY, FEBRUARY 7, i^ 



EARTHQUAKES. 

 Les Tremblements de Terre. Par F. Foiiqud, Membre 

 de rinstitut (Acaddmie des Sciences), Professeur au 

 College de France. (Paris : J. B. Bailli^re et Fils, 

 1888.) 



IN the introduction to his little volume on earthquakes, 

 Prof. Fouqu<? observes very justly that it is onlyin recent 

 years that seismology has begun to shape itself to the lines 

 of an exact science. Its students have of late concentrated 

 their attention on questions susceptible of direct attack 

 by observation and experiment. The older seismologists 

 made the mistake of attempting to take the citadel by 

 storm, and failed. The younger school of investigators, 

 proceeding more gradually, have at least succeeded in 

 showing how enormously complex the problem of earth- 

 quakes in their origin and propagation really is. The 

 older seismologists were for the most part men with little 

 knowledge of mechanics, and their fundamental mistake 

 was that they under-estimated the difficulty of the prob- 

 lem in its mechanical aspect. Setting to work with a pre- 

 conceived and quite false idea of its simplicity, they used 

 such observational data as were at their disposal to build 

 up an elaborate structure of inference and hypothesis — 

 a structure very ill adapted to bear the shock of the first 

 earthquake that formed the object of scientific measure- 

 ment. The foundation on which the new school builds 

 its science is exact seismometry ; and so far, little, if 

 anything, more than the foundation is laid. It is less than 

 ten years since instruments of precision were introduced, 

 capable of giving complete information as to the manner 

 of motion of the ground. We now have sufficiently full 

 and exact knowledge of the nature of the motion which 

 takes place at one or another point of the earth's surface 

 in the affected region while an earthquake is going on. 

 The elaborate recording seismographs which have been 

 brought to something like perfection by a few enthusiastic 

 workers in Japan have analyzed this motion as completely 

 as can be desired. But beyond this we as yet know 

 next to nothing with any certainty about the real character 

 of an earthquake. The relation which exists between the 

 motion at one po'it and that at another, the manner of 

 the motion below the surface, the transformations which 

 the seismic waves undergo en route, are subjects hardly 

 touched ; and no seismometric observations have as yet 

 been made, in a single case, from which conclusions may be 

 drawn with any certainty as to the position of the origin 

 and the nature of the originating disturbance. These are 

 matters which used to be glibly settled by reference to a 

 few projected stones or cracked walls, or to the stoppage 

 of some village clocks : if the new seismometry has 

 not yet thrown much light on them, it has at least shown 

 how gross was the former darkness. 



It is, then, not a little surprising to find Prof. Fouqu^ 

 write a book on earthquakes without so much as a chapter 

 on seismometers. He excuses himself from taking up this 

 branch of the subject on the curious ground that its ex- 

 treme importance makes it deserve a special treatise, and 

 further, that, as the instruments are being improved from 

 Vol. xxxix.— No. 1006. 



day to day, " nous pensons que la description des sdismo- 

 graphes et microsdismographes gagnera singuli^rement 

 k n'etre exposde en ddtail que dans quelques anndes." 

 Readers of Nature, who have had the opportunity from 

 time to time in these pages of seeing what present-day 

 seismographs are able to do, will scarcely agree with the 

 author ; and granting, as we very well may, that many 

 improvements have still to be made, the results already 

 achieved in exact seismometry are surely such as not 

 only to justify but to demand some description of these 

 appliances in any new treatise on earthquakes. 



In fact, however, M. Fouqud has been better than his 

 word, for, in speaking of the " intensity " and the com- 

 ponents of earthquake motion, he has given some slight 

 account of seismographs and seismograms. But theaccount 

 is far from adequate, and is not free from serious errors. 

 We find the old fallacy restated, that the position of the 

 " epicentre " can be determined by observation of the 

 azimuths of the oscillations ; that the bearing of the origin 

 is given by the direction in which pendulums are set 

 oscillating or objects are thrown down. Anomalous cases 

 are spoken of, but not a word is said to explain that the 

 cases which are styled anomalous form, not the exception, 

 but the rule, because the chief oscillations are in general 

 not of the normal but of the transverse type. With regard 

 to the mechanical theory of seismographs, the author is 

 completely at sea. It is now well known that the first 

 essential in seismometry is to secure a point of reference 

 by having a steady mass pivoted or hung in nearly 

 neutral equilibrium ; that a stably-hung mass like a 

 common pendulum will not do, because it acquires 

 oscillation through the more or less close agreement 

 between its period of free swing and the period of the 

 successive seismic impulses. Nothing could be worse 

 than a pendulum with the period of which these impulses 

 happened just to agree. Nevertheless, M. Fouqud says, 

 without hinting dissent : — 



" M. Cavalleri admet, d'apres ses observations, que, 

 dans un tremblement de terre, le meilleur pendule au point 

 de vue de I'indication des intensitds est celui dont les 

 oscillations sont synchrones avec la durde de I'ondulation 

 du sol; les pendules i fil long donnent le tracd le plus 

 dtendu quand les mouvements du sol sont lents ; I'inverse 

 a lieu quand les vibrations sont rapides. Par consequent, 

 pour obtenir un tracd, qui soit I'image aussi fiddle que 

 possible de I'intensite de la secousse, il faudrait avoir une 

 sdrie de pendules d'indgale longueur, et considdrer 

 exclusivement, parmi les traces obtenus, celui qui offi-e les 

 dentelures les plus allongdes." 



Nothing could be more complete than the misappre- 

 hension shown in this last sentence. Other indications, 

 lead one to conclude that the author's acquaintance with 

 seismometry is not intimate, and that it has not been 

 formed at first hand. His references to original sources 

 of information are rare. He gives a fairly good account 

 of the work of Milne and Gray on the measurement of 

 the speed of propagation of artificial disturbances through 

 the soil — a subject the author has himself investigated — 

 but of the work of Ewing in measuring natural earth- 

 quakes, and of the continuation and extension of it by 

 Sekiya, he is apparently ignorant. Ewing's horizontal 

 pendulum seismograph is not described, and his duplex 

 pendulum seismograph, although mentioned, is wrongly 

 classed as an instrument that records the phases of the 



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