Earthquake at the Philippine Islands. 55 
After this first movement there were two more shocks at the 
end of an hour and a half. On the 15th and 16th no percep- 
- tible shocks occurred ; and on the 17th, only two small shocks. 
On the 18th, at 12" 40’ Pp. M., occurred the great shock—one 
of oscillation and also of “trepidation,” and spoken of com- 
monly at the time as one of rotation. Its duration was 70 sec- 
onds. The movements of the pendulum were so many and 
various that it is not possible to indicate them all, and we 
limit our descriptions to the principal directions and the ampli- 
tudes of the same. For the rest the reader may refer to fig. 2. 
It may however be stated that, in our opinion, only the great 
oscillation from E. to W., which was the most regular and 
without violent shocks, corresponds to the actual inclinations of 
the disturbed buildings toward the west. 
1. Maximum oscillation, from S. 85° E. to N. 85° W. (68’, 
the direction of most intense oscillation); amplitude of the 
greatest oscillation in this direction 22°—or 11°, E. and 11°, W. 
2. Maximum oscillation, from S.W. to N.E. true; amplitude 
19°, but 10° 10’ to the S.W., and 8° 50’ to the N.E. 
. Maximum oscillation, from N. 4° W. to S. 4° E.; ampli- 
tude 16°, but 9° N. and only 7° S., whence it appears that the 
impulse was from the north to the south. The index of the 
vertical seismometer was moved 34™™ from its position. 
rom this time there was an uninterrupted series of small 
shocks, until the 20th at 8" 40’ p. M., and then occurred a 
repetition of extraordinary violence, though only movements of 
oscillation and trembling (“trepidation)” occurred. Its dura- 
tion was 45 seconds. The direction of oscillation was from S. 
received a new impulse which not only destroyed the velocity 
that it had acquired in its descent, but forced it to go a second 
ae Impulse. The vertical pendulum was moved 
'rection SS (see figure) was one of intense oscillation. 
ina N.E. and S.W. direction. At 10" 40’ p. M., occurred a 
second violent repetition, which lasted 55 seconds; and this 
shock had its peculiarities. In the preceding, the focus of most 
Intense seismic radiation was in the second quadrant; in this, it 
began in the E. true, yet with much less intensity than before ; 
and the focus before observed in the first quadrant continued 
Operate but with greater violence. In figure 4, we observe 
