THE TIME AND ORIGIN OF THE SHOCK. 9 
of the epicentrum; and z,, the depth of the centrum. The point lies exactly opposite 
the Golden Gate. 
If, instead of a velocity of 7.2 km./sec. we had used 6.5 or 8, the position and time 
of the shock would not have been altered beyond the limits of error indicated above. 
A smaller velocity would have led to a deeper centrum and earlier time; a greater veloc- 
ity would have had the opposite effect. 
THE VIOLENT SHOCK, 
The violent shock is the most important part of the earthquake, both on account of 
its destructive effects and because it alone could have affected distant seismographs. 
Indeed, Victoria is the only distant station where the first motion was recorded and it is 
the nearest seismographic station beyond the limits of sensible motion. Its distance was 
1,156 km. from the origin and the disturbance was perceptible to a distance of 550 km. 
A large number of clocks were stopt by the strong motion; and one would naturally 
look to them to get the exact time of its occurrence. Professor Marvin has collected 
together all information regarding these clocks. For the great majority the time of the 
stopping is only known to minutes, and even then the differences between the various 
clocks are so great as to make the resulting average of very little value; it is therefore 
not necessary to give here the times recorded by all of them. We fortunately have 
observations from four stations which seem to be very reliable. 
First, San Rafael: Two standard clocks were stopt; one the standard clock of the Time 
Inspector of the North Shore Railroad, stopt at 5" 12™ 35%; the other, belonging to the 
night operator of the Railroad, stopt at 5" 12™ 30%. Also a clock, belonging to the 
Western Union Telegraph Company, which sends out the time, stopt, the time being 
55 13™; as the seconds are not given it is probable that they were not observed. This 
time must, therefore, be neglected; it is manifestly too late. The first two clocks are 
supposed to be very accurate and to be checkt every day at noon. The average of their 
fameis 5>, 12") 32.5". 
Second, Mare Island: Two of the astronomical clocks, under the charge of Prof. 
T. J. J. See, stopt respectively at 55 12™ 35° and 58 12™37°%. A third clock which is 
electrically corrected every day, but is not a standard clock, stopt at 5" 12™ 33°. 
Professor See thinks the best time is the average of the two astronomical clocks, namely, 
5P.12-36% 
Third, Berkeley: The astronomical clock at the Students’ Observatory, under the 
charge of Prof. A. O. Leuschner, stopt at 5® 12™ 39°. 
Fourth, Mount Hamilton: The only clock that stopt was a small one in the Director’s 
office, the correction for which was not accurately known; it stopt at 5" 12™ 528; we 
can not put any reliance on the exact time it gives. The shock began at Mount Hamilton 
at 55 12™ 12° but the strong shock occurred at 5" 12™ 45%. This is attested by the 
observation of Mr. Aitken and also by the fact that the Ewing seismograph was set in 
motion at that time. This seems the most accurate record we have of the time of the 
arrival of the heavy shock at a station near the origin. 
The sidereal clock at the Chabot Observatory, Oakland, stopt at mean time 5> 12™ 
51%. The mean time clock stopt at 55 14™ 48°. Professor Marvin, however, has 
pointed out that the delicate gravity escapement of the latter might easily be thrown 
out of adjustment by the disturbance and thus allow the clock to race. It is evident that 
we can not take into consideration the time given by it. The sidereal clock, stopping 14 
seconds later than the clock at Berkeley, may perhaps have been stopt, restarted, and 
stopt again by the shock. At any rate it is certainly too late and must be neglected. 
One clock at Ukiah may have been stopt and restarted; it was going after the shock, 
