OBSERVATORIES AND THE DATA OBTAINED. 67 
There were no regular time marks on the seismograms, but certain marks have been 
made artificially, which enable us to make fair estimates of the times. 
The motion begins very gently at 17" 02°; at 17™ 32° astronger movement occurs, 
which dies down in a little over a minute; it is possible that this is the true beginning of 
the first preliminary tremors; at the beginning and soon after the end of this movement 
there were east-west Jars which shook the recording drum and caused an overlapping of 
the record. The beginning of the second preliminary tremors is about 21™ 06° with an 
amplitude of 11 mm. This continued with some irregularity until 22™ 32%, when the 
marker went beyond the limits of the paper, 7.e., with an amplitude greater than 65 mm. 
The strong motion lasted about 14.5 minutes to 13" 36™, and then the movement con- 
tinued with small amplitude and occasional reénforcements until about 15° 17™. It is 
not unlikely that the times at Sitka are all about a half-minute too late; this correction 
would make them accord better with observations from other stations in drawing the 
hodographs. 
The damping is entirely negligible; and the solid friction is not large. The period 
during the large motion is practically that of the pendulum, so that it is impossible to 
estimate the magnification, or the actual movement of the earth. 
The question arises: Did Sitka, like Victoria, record the early slight shock or only 
the violent shock? If it recorded the early shock, the average velocity from the origin 
would have been 2,316/304 = 7.63 km./sec.; but since this shock reached Victoria at 
14.2™, it must have progressed from that neighborhood to Sitka at a rate of about 
(2,316 — 1,180)/ 170 = 6.7 km./sec. We can not believe that the disturbance advanced 
for the first half of the distance to Sitka at a rate of 8.8 km. /sec., and for the second half 
only ata rate of 6.7 km./sec. We are, therefore, obliged to believe that Sitka, which 
recorded the component at right angles to that recorded at Victoria, really recorded 
only the violent shock; the velocity for which then becomes 2,291/274 = 8.36 km./sec., 
which is about the velocity we should expect. 
TACUBAYA, D. F., MEXICO, 
Observatorio Astronomico Nacional. Senor F. Valle, director. 
Lat. 19° 24’ N.; long. 99° 12’ W.; altitude, 2,280 meters; distance, 27.70° or 3,081 km. ; 
chord, 3,050 km.; direction, 8. 54° E. 
Foundation, alluvium. 
Seismograms, sheet No. 9. 
The instruments used were Bosch-Omori horizontal pendulums, two horizontal com- 
ponents; mechanical registration on smoked paper. 
(1) North component: T',, 17.3 seconds; V,15; J, 1,120 meters; M,10kg.; L, 75 cm. 
(2) East component: 7, 17.6 seconds; V, 15; J, 1,160 meters; M, 10 kg.; L, 75 cm. 






First PRELIMINARY|SECOND PRELIMINARY| REGULAR | PRINCIPAL Mike 
REMORS. TREMORS. WAVES. Part. AMPLITUDE. |PERIOD. 







™m. 8. mm. 8. m. 8. m. 8. ™m. 8, mm, 
(1) North component 17 58 22 50 25 08 | 26 40/26 45| 50+ |15-25 
(2) East component 17_58 22 54 26 06 | 25 30 | 26 80 + 
Average . . . 17 58 22 52 25 37 | 26 05 
Interval 5 30 10 24 1S 09 1S a7 






The second phase is much stronger on the east component than on the north component, 
which is due to synchronism of periods; and the long waves begin more definitely on the 
east component. 
