Page:Science vol. 5.djvu/217

 Mabch 9, 1885.] 8Cri

word, the subject for research is whether one of the Itist two theories will apply lo the case in question. You will notice, moreover, tliitC each of Ibese theories presumes a geological cause. It is in part, I tliink, this idea of the connection between earthquakes and tbe muTements far below the surface, that has Inllu- eticed the Academy of n^iences in choosing a geolo- gist to examine the phenomenon.

In ray turn, — and for the same reason as the Academy at iiclenceK, — I have taken geologists as collaborators. Those who accompany me are Messrs. Michel L^vy and Marcel Bertrand, members of the geological survey of France, and mining en- gineers of great competeoce. The third who accom- panies me is Professor Barrola. of the Faculty of science at Lille, an eminent geologist, who is well acquainted with the Spanish soil.

1 have, then, as my associates, three geologists, perfectly competent to study all the facts that are usually investigated In earthquakes, — the propaga- t)o[| of the motion, the direction of the sliock, and the place of greatest intensity. They are also capa- ble of detennlning 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 superlicial 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 siiall endeavor to determine.

We wish, from the study of the sugierQcial deposits, tA 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 shoclc originates. We have two methods for this. One, which is founded upon very precise and delicate observations, has been proposed Ijy Mr. Seehach : it la based upon the determi- nation of a series of points, in which the oscillations are felt at the same moment. These observations are extremely difBcult to obtain,

There Is another, older method, due to the English physicist. Mallet. Tbe 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- glD of the shock.

The methods of which t have spoken are not purely theoretical : they have been applied Ave 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 lost earthquake at lachin, It bos 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- "~ " !i there Is certainly a considerable range;

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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 observation* the constitution of the eartit 3,1 this point, we shall have obtained a datum ex- tremely Important, and we may be able to accpt 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. 1 hope that we shall obtain satisfactory results. 1 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.

SEISMOLOOICAL NOTES.

Tbg earthquakes of the last year lo England have, like those in this country, aroused an Interest in sels- mometry ; and the committee of the Scottish meteor- ological society, who have charge of the Hen Nevis observatory, have asked Professor Ewing (whose work in Japan we recently noticed Jvol. Iv. p. Gie), and who Is now professor of engineering in University college, Dundee) to Institute earthquake observa- tions on the top of Ben Nevis. Professor Kwing has received a grant of a hundred pounds from the com- mittee controlling the government grant for scientific investlgatiiin, and will proceed to set up apparatus to detect, and probably lo record, minute earth-tremon, 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 seiamoacope. In Nature, vol. xxxl. p. 202, Hr. Ellis of the Royal observatory at Greenwich states that the continuous photographic records of the declination and horii:oii- tal force magnetometers both show a simultaneous disturbance, different from the ordinary magnetic disturbances, occurring on the evening of Dec. 26, 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 lo bo little if any reason to doubt that the unusual disturbances recorded were caused by the swinging of the magnets on their suspending flbres, 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 uKimuths at right angles, in addition to the present torsional swing as [uagnetometers, these instruments could, perhaps, be made very sensitive seismoscopcs as well, and the accuracy of the lime-reconl would only depend upon

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