Page:Science vol. 5.djvu/218

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��SCIENCE.

��IVoL. v.. No.

��the velocity given Lo ihe strip of pholograpliic paper. Of couree, u aeismomelem^ tile; would be as worlbless aa all stable pendulumi must be ; but aa Keinmotcopes, the; might be quite sensitive, and the expense and requisite attention need not add greatly to that ai- rend; necessary with the raagnetomeler.

In Japan, Professor Milne keeps up his active work in seiamology. During the lost «uiumer, he spent five days on the top of Fujiyama, attempting lo detei^t illuriial ciiaiiges in the level of the ground. The re- sults have not yet been published. This mountain — a srundertuity symmetrica] volcaniccone.aboullwelve thousand feet high, and the most striking object in all Japan — is the one on whose summit Professor Mendenhall maile a determination of the force of gravity and of the values of the magnetic elements; and It will always be an interesting point for scien- tific observations of all liiuds, rising as it does In complete Isolation out of a plain.

In vol. vii. part 2, of the TVaiuacllons of Ihe seis- mologlual society of Japan, Professor Milne con- tributes a paper upon three hundred and eighty-seven earthquakes observed In northern Japan between October, 1881, and Ocl^iber, I6S:}. A map Is given for everjr quake, showing by Its colored portion the approximate area covered by the shock, as determined by Professor Milne's system of tracking down earth- quakes by a system of postcards distributed to all important places In the bands of observers who send in weekly reports of the occurrence or non-occur- rence of any disturbances. In this way Professor Hllne has hod the northern part of Nippon and the southern part of Tezo covered for several years with a network of forty-five observers, besides those in Toklo and Yokohama. At five of these stations quite accurate time-observations of the disturbances were frequently oblalued by the help of good clocks compared several times per week with the daily tele- graphic noon signal from Tokio. A catalogue of the individual observations of eachuf the three hundred and eighty-seven shocks is also given. Some of the results are worth noting. As regards geographical distribution, it Is remarkable that only two out of the three hundred and eighty-seven shocks appear lohave extended to the west of the range of mountains running up the western side of the Island of Nippon, being apparently stopped by that barrier, while alwm elghty'four per cent seem to have originated either out under the ocean or very near it on the eastern side of the islands. Commenting on this, Professor Milne says, —

"The district which is most shaken is the flat al- luvial plain of Musashi following the line of the river Tonegawa. . . . Tbia area forms one of the flattest parts of Japan. The large number of earthquakes which have been felt on llie low ground, and the com- paratively small number which have been fell in the mountains, Is certainly remarkable.

" It must also be otmerved, that, in the immediate vicinity of active or extremely recent volcanoes, the seismic activity has been small. ... It may also be remarked that the side of Japan on which earth-

3nakes are the most frequent is the slile which slopes own steeply beneath an ocean which at a hundred

��and twenty miles from the eoai

two thousand fathoms, whilst

of the country, at the same distance from 'the shore,

the deptli is only about a hiudred and forty fatbc

Another point not lo be overlooked is the fact that

the district where earthquakes

Is one where (here is abundant evidence of

and rapi<l elevation.

" In all these respects the seismic regions of Ji hold a close relationship to similar regions in S< America, where we have earthqu^es orlelnatIq(3 beneath a deep ocean at the foot of a steep slope on the up|)er paru of which there are numerous volcanic vents, whilst, on the side of this ridge opposite the ocean, earthquakes are rare. With re^rd lo the Musaslii area, it may also be remarked that sediment*' brought down by numerous rivers from the higbi ~ parts of the country are accumulating on It at

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��The distribution of the three hundred and eighty- seven earthquakes for the four quarters of the ;e«rs was as follows, — January -March, 165; April- June, 70; July-September, 30; October-December. B3,— thus confirming the greatest activity in the coldest, and least in the hottest, months of the year, which had been shown before for the Tokio district alon* fur a long period of years.

With respect to the measurement of the motion the ground, most of the facts deduced by Profi Milne are substantially the same as those rized by Pro(es«or Ewing in his memoir referred' above. The following, however, which is partly, least, new, deserves quotation here: -

"Inasmuch as it will be observed that dlfTereni Instruments give different results for the same earth-

3uake, in order tbat the reader may not n^ard such iagrsms as conflicting, the following results, which have been obtained from the earthquakes here re- ferred to, and which have been confirmed by many observations made subsequently, may be enumer- ated:

" 1. An ordinary earthquake, although having a general direction of propt^ation, has at a given point many directions of vibration. If there is a decide ' shock in a disturbance, this particular movemi may be indicated in the same manner at adjaci stations.

"2. The amplitude of motion as observed adjacent stations, even if only a few hundred U apart, may be extremely diflerenL

";i. The [leriod of motion may vary like the plitude, the instruments being In all cases as simll as it is possible to construct them.

" At present I am carrying on observations by

of three similar instruments placed at the con

a triangle the sides of which are about eight hundi feet iulength. When these instruments are side side, they practically give nimilar diagrams. At thL_ present positions, they always give d^erenl diagr&ms. If these Instruments were in the hands of distinct observers, each of these observers would give a lotallj different account of the same eartliquake. Judging from the quick period and large amplitude of mo- tion always observed at one particular corner of my triangle, lean say with confidence that at this comer there might l>e sufficient motion to shatter a build- ing, whilst at the oilier corners similar huildiuf^ would not be damaged."

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