March 6, 1885.] 



SCIENCE. 



197 



commission's work in attractive form. It is some- 

 times even a little amusing to notice the purely sci- 

 entific treatment that these distressing calamities 

 receive; for, just as an old surgeon will describe a 

 terrible operation as a ' beautiful case,' so Forel writes 

 of a violent shock as ' ce beau tremblement de 

 terre.' Spain must advance far beyond its pres- 

 ent superstitions before it can have so calm and 

 judicial a commissioner. The classification that 

 was early adopted is an important matter, and, in the 

 present stage of the study in this country, deserves 

 quotation in full ; for, in any statistical comparisons, 

 it is important that the facts on which they rest 

 should be recorded on similar scales. The first prin- 

 ciple is the grouping of the fainter antecedent and 

 subsequent tremors with the more violent shocks, as 

 making parts of a single disturbance; and, although 

 this is generally well advised, it sometimes leads to 

 including shocks {secousses, stossen) that occurred 

 during ten or more days as parts of a single earth- 

 quake (tremblement, beben). Thus, in 1880, there 

 were sixty-two tremors or shocks in twenty-one 

 earthquakes; and in 1881 the numbers were one 

 hundred and sixty-three, and thirty-seven for Switz- 

 erland alone. The intensity of shocks is measured 

 on the Kossi-Forel scale, as follows : — 



1. Yery faint; recorded by a single seismometer; 

 noticed only by practised observers. 



2. Registered on several seismometers of different 

 construction; noticed by a few persons at rest. 



3. Duration or direction noted ; felt by a number 

 of persons at rest. 



4. Felt by persons while moving; shaking of mov- 

 able objects, doors, windows; cracking of ceilings. 



5. Felt by everyone; furniture shaken, and some 

 bells rung. 



6. Sleepers awakened; general bell-ringing, clocks 

 stopped, visible swaying of trees; some persons run 

 out of buildings. 



7. Overturning 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 

 fissures and mountain-slides. 



In order to obtain a measure of the ' value ' of the 

 earthquake in which all 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 of five, fifty, one hundred 

 and fifty, and five hundred kilometres; and the weak, 

 medium, and strong accessory tremors are counted 

 separately (n, n', n"). Then the total value of a dis- 

 turbance is V — (Intensity scale X area scale) + n 

 + 2n' + ?jn". This is evidently a useful method of 

 combining and giving weight to the various pecul- 

 iarities of an earthquake, but it has a manifest inac- 

 curacy coming from the inequality of 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 

 shock. 



The numerical results thus far announced may be 

 briefly summarized: 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 the day; no sufficient connec- 

 tion is made out between the attitude 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 be 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 is 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 far 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 the 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 centre, are 

 properly referred to a single origin: examples of such 

 are found in 1879, vii., and 1880, i., ix., xiii., and xx. t 

 of Forel' s lists. But in a few other cases the succes- 

 sive shocks must be referred to different centres, 

 which travel or ' migrate ' along a line that is natu- 

 rally supposed to mark a yielding fissure. 1879, v., 

 and 1880, viii., belong to this interesting class. Still 

 more peculiar is the interpretation given by Heim to 

 number xlvii. of his list (June 28, 1880). 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 too 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 succes- 

 sive breaking or slipping of a long fissure, from which 

 earth-waves spread out laterally with 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 a tempting one, and, if confirmed by 

 similar results in the future, will be an important 

 contribution to seismology. 



The statistical results that will, after a few decades, 

 be gathered from these uniformly recorded observa- 

 tions, will be of especial value; and the further de- 

 velopment of the connection that has been surmised 



