C. G. Rockwood, Jr.—Japanese Seismology. 475 
second N.N.E. or N.E. It is interesting to note that the pend- 
ulum records of the Luzon* earthquake of July, 1880, show 
likewise the presence of several wave directions in azimuths 
not widely different from those here state 
trary to that of the hands of a watch. As to the cause of such 
rotation, Mallet’s explanation, Hh aa attributes it to the verti- 
cal through the center of gravity not coinciding with the center 
of friction, is rejected as not in accord with the great prepon- 
derance of rotation in one direction, and another at ielees 
suggested by T.. Gray is offered, to this effect: If any columnar 
object having a rectangular base is acted u upon i a force 
parallel to either side or to either diagonal of the rectangle, it 
will tend to overturn without rotation. If, however, the force 
has any direction other than these, there will be a ten ency to 
rotation in a direction determined by the relation of the line of 
force to that diagonal which lies nearest to it. If the rectangle 
be divided into eight equal triangles by the two diagonals and 
two medial lines parallel to the sides, and the alternate trian- 
gles be shaded, it will be seen that the rotation will be in one 
or the other direction, according as the direction of the force, 
falls in a shaded or an unshaded triangle. The direction in 
which a stone is found to have been twisted will then enable 
us to assign limits to the direction from which the impulse that 
moved it must have come, and will thus serve to indicate the 
direction of the earthquake shock. With regard to the earth- 
quake in question, the direction inferred in cis way from the 
numerous twisted grave-stones, agrees in general with the 
instrumental indications noted above. 
This earthquake was sensibly felt over an area included 
within a radius of one hundred and twenty miles. From the 
directions of the shock as observed at Tokio and Yokohama, 
and from other considerations, the author concludes that the 
salepeae origin of this earthquake was nearly equidistant from 
okio and Yokohama, pa somewhat to the east of them, under 
the eastern shore of o bay. Indeed many of the recent 
earthquakes in Japan ps to come from that region. The 
geological characteristic of that district is beds of voleanic tufa 
and breccia very much faulted and contorted in the southern 
extending 1,500 miles southward into the Pacific through the 
Bonins ; and it is also on another line of volcanoes, 3,000 miles 
* This Journal, III, vol. xxi, p. 52, January, 1881. 
