Page:Science vol. 5.djvu/215

 SCIENCE.

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��commiMlon's work lu attractive fiirni. It is aonie- UineB even a little amusing lo notice the purely sei- entiBe IreaUnent that these distressing calamities receive; for, just as an old surgeon will describe a terrible operation as a ' beautiful case,' so Furei writes I a violent shock as ' ce beau trembleroent de Spain must advance far l>e;ond Its pres- t luperslitions before it can liave so calm and (dicial a commissioner. The ctasslHcatlon that la earl; adopted 1b hu important matter, and, in the present stage of the study in this country, deserves quotation in full; for. In any statlatical comparisons, ft Is important iliat the facts on which they rest should be recorded on similar scales. The first prin- ciple is the grouping of the fainter antecedent and subseiguent tremors with the more violent shocks, as making parts of a single disttirbance; and, although this Is generally well advised, it sometimes leads )o including shoclis (oecouisen, stossen) that occurred during ten or more days as parts of a single earth- quake Itremblenienf, beben). Thus, In IS80, there were sisty-two tremors or shocks In twenty-one earthquakes; and In 1861 the numbers were one hundred and sixty-three, and thirty-seven forSwitz- erland alone. The Intensity of shocks is measured on the Rossi-Forel scale, as follows; —

1. Very faint; recorded by a single seisniomater; noticed only by practised observers.

2. Registered on several seismometers of different

I construction; noticed by a few persons at rest. 3. Duration or direction noted; fell by a number if persons at rest. [4. Felt by persons while moving; shaking of mov- ible objects, doors, wiiidows; cracking of ceilings. I S. Felt by every one; furniture shaken, and some Mils rung. P 6. Sleepers awakened; general bell-ringing, cloclcs Hopped, visible swaying of trees; some persons run oat of buildings.

7. Overtnnilng of loose objects; plaster falling, general fright; buildings not seriously injured.

8. Chimneys falling; walls cracked.

►9. Partial or total destruction of buildings. 10. Great disasters; overturning rocks, forming aurei and mountain-slides. In order to obtain a measure of the ' value ' of the earthquake In which alt its elements are Included, the area affected and the number of accessory shocks must also be considered. For Switzerland, the areas are grouped by diameters ot five, fifty, one hundred and &fty, and Hve hundred kilometres; and the weak, medium, and strong accessory tremors are counted separately (n, 11', n"}. Then the total value of a dis- turbance la V = (Intensity scale x area scale) -I- n + 2n' + 3n". This is evidently a useful method of coubinlog and giving weight to the various pecul- iarities of an eartliquake, but It has a manifest Inac- curacy coming from the inequality ot the divisions in the scale of Intensity. Great earthquakes would not be given their deserved superiority over small ones in such a measurement. It would be Improved by squaring the intensity number of the principal

��ulls thus far announced may he briefly sun)mari7.ed : they give a moderate winter maximum, thus agreeing with Volger's studies of some years ago; a strongly marked preference for the night hours, with a maximum between two and four in the morning, while the minimum is from noon to two o'clock in Ihc day; no sufficient connec- tion is made out between llie altitude of the moon and the occurrence of shocks; and the south-western corner of the country has had twice as many earth- quakes as any other, but no general map showing distribution has yet been published,

There seems to he no dissent from the opinion that these shocks are in no way of volcanic origin: they are by all regarded as evidence of continued struc- tural disturbance and growth of the Alps. There is no appearance of volcanic action, but evidence of lateral crowding la afforded by every valley that exposes sections of distorted rocks on its sides. The distortion may be slow and uniform, and evenly dis- tributed through the rocks, especially when Tar below the surface, under the heavy weight of overlying strata; and then it is probable that no disturbance would be felt above. But it may also be irregular by fits and starts, as tjie crushing stress accumulates to the limit of the rocks' strength, which snap asunder as the limit Is passed; and the tremor thus produced is known on the surface as an earthquake. The migration of shocks gives valuable confirmation of this view. Some earthquakes, composed of a num- ber of accessory shocks having a common centra, are properly referred to aslngie origin: examples of such are found in 1870, vll., and 1880, 1., ix., xlii.. and xx., ot Forel's lists. But in a few other cases the succes- sive shocks must be referred lo different centres, which travel or 'migrate' along a line that la natu- rally supposed to mark a yielding fissure. lS7fl, v., and 1880, viii.. belong to this interesting class. Still more peculiar is the Interpretation given by Helm U> number xlvli. of his list IJune 28, 18S|. The obser- vations of this earthquake showed only a moderate velocity of propagation (112 to 204 metres a second) in the direction of the longer diameter of the region affected, and this Is regarded as loo small for the advance of an elastic earth-wave. Moreover, the local directions of the shock, agreeing fairly well among themselves on either side of the longer diam- eter, did not agree with the direction of the extension of the disturbed area in time. It was therefore sup- posed that the disturbance resulted from the auccea- sive breaking or slipping of a long fl»sure, from which earth-waves spread out laleraily wllh normal veloci- ty; thus showing the migration of the focus quickly accomplished in a simple earthquake, much as it had been implied by the more deliberate shifting of the successive shocks in complex disturbances. The explanation Is n teiupllng one, and, if confirmed by similar results In the future, will be an Important contribution to seismology.

The statistical results that will, after a tew decades, be gathered from these uniformly recorded observa- tions, will he ot especial value; and the further de- velopment of the connection that has been surmised

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