JMA.KOH 6, 1885. 



SCIENCE. 



199 



word, the subject for research is whether one of the 

 last two theories will apply to the case in question. 

 You will notice, moreover, that each of these theories 

 presumes a geological cause. It is in part, I think, 

 this idea of the connection between earthquakes and 

 the movements far below the surface, that has influ- 

 enced the Academy of sciences in choosing a geolo- 

 gist to examine the phenomenon. 



In my turn, — and for the same reason as the 

 Academy of sciences, — I have taken geologists 

 as collaborators. Those who accompany me are 

 Messrs. Michel Levy and Marcel Bertrand, members 

 of the geological survey of France, and mining en- 

 gineers of great competence. The third who accom- 

 panies me is Professor Barrois, of the Faculty of 

 science at Lille, an eminent geologist, who is well 

 acquainted with the Spanish soil. 



I have, then, as my associates, three geologists, 

 perfectly competent to study all the facts that are 

 usually investigated in earthquakes, — the propaga- 

 tion of the motion, the direction of the shock, and 

 the place of greatest intensity. They are also capa- 

 ble of determining the relations which exist between 

 the superficial action of an earthquake and that 

 which may be going on at great depths. Geologists, 

 when they travel over the surface of a piece of 

 ground, see not only the superficial beds, but, by a 

 sort of instinct, they divine the character of the 

 deeper extensions. Sometimes they are mistaken, — 

 they are not infallible, — but still, in the most cases, 

 they are able to determine the constitution of the 

 deep strata. This, then, is one special point which 

 we shall endeavor to determine. 



We wish, from the study of the superficial deposits, 

 to deduce its geological structure at a certain depth. 

 On the other hand, with the means which we possess 

 to-day, it is possible to determine approximately the 

 depth from which an earthquake shock originates. 

 We have two methods for this. One, which is founded 

 upon very precise and delicate observations, has been 

 proposed by Mr. Seebach : it is based upon the determi- 

 nation of a series of points, in which the oscillations 

 are felt at the same moment. These observations 

 are extremely difficult to obtain. 



There is another, older method, due to the English 

 physicist, Mallet. The system of observations pro- 

 posed by him is based upon the examination of the 

 cracks in the land after an earthquake. These frac- 

 tures are, in nearly every case, normal to the di- 

 rection of the shock; and, when one studies them 

 carefully, the direction of these normals is sufficient 

 to fix their points of convergence, and hence the ori- 

 gin of the shock. 



The methods of which I have spoken are not purely 

 theoretical : they have been applied five or six times by 

 Germans, Italians, and English; but, unfortunately, 

 the French have not yet used them. They have 

 given very interesting results ; as, for instance, in the 

 last earthquake at Ischia, it has been shown that the 

 cause of the concussions came from a depth of from 

 twelve hundred to eighteen hundred metres at the 

 most. Between twelve hundred and eighteen hun- 

 dred metres there is certainly a considerable range; 



but one would have expected to find that the shock 

 came from a much greater depth. Consequently 

 much is already accomplished, when we can limit the 

 origin of the phenomenon to a space so restricted. 



I said that we were able to apply these two meth- 

 ods, the one certainly, the other probably. We may 

 thus ascertain the depth of the earthquake's centre. 

 If, on the other hand, we are able to determine by 

 geological observations the constitution of the earth 

 at this point, we shall have obtained a datum ex- 

 tremely important, and we may be able to accept one 

 of the two theories, or so to limit one or the other as 

 to make it agree better with the facts. 



These are the objects of our mission, these the 

 things we count on accomplishing. You will see 

 that it is very simple. I hope that we shall obtain 

 satisfactory results. I do not dare to promise that 

 we shall; but I do promise you that we shall study 

 Andalusia, or a portion of this province, with care, 

 and that we shall bring back data of geological 

 interest and importance from this very curious 

 country. 



SEISMOLOGICAL NOTES. 



The earthquakes of the last year in England have, 

 like those in this country, aroused an interest in seis- 

 mometry ; and the committee of the Scottish meteor- 

 ological society, who have charge of the Ben Nevis 

 observatory, have asked Professor E wing (whose work 

 in Japan we recently noticed [vol. iv. p. 516], and 

 who is now professor of engineering in University 

 college, Dundee) to institute earthquake observa- 

 tions on the top of Ben Nevis. Professor Ewing has 

 received a grant of a hundred pounds from the com- 

 mittee controlling the government grant for scientific 

 investigation, and will proceed to set up apparatus to 

 detect, and probably to record, minute earth-tremors, 

 and also slow changes of level of the ground. 



In connection with the recent Spanish earthquakes, 

 it is interesting to note that we have accidentally 

 brought into prominence a new kind of seismoscope. 

 In Nature, vol. xxxi. p. 262, Mr. Ellis of the Royal 

 observatory at Greenwich states that the continuous 

 photographic records of the declination and horizon- 

 tal force magnetometers both show a simultaneous 

 disturbance, different from the ordinary magnetic 

 disturbances, occurring on the evening of Dec. 25, a 

 few minutes after the reported time of the severe 

 earthquake in Spain on that date. No ordinary mag- 

 netic disturbances were recorded on this and neigh- 

 boring dates, and the earth-current registers showed 

 no change; so that there would seem to be little if 

 any reason to doubt that the unusual disturbances 

 recorded were caused by the swinging of the magnets 

 on their suspending fibres, due to the shaking of the 

 points of suspension by the Spanish earthquake. If 

 some method were devised of photographing the lat- 

 eral swing of the magnets in two azimuths at right 

 angles, in addition to the present torsional swing as 

 magnetometers, these instruments could, perhaps, be 

 made very sensitive seismoscopes as well, and the 

 accuracy of the time-record would only depend upon 



