PERIODS AND AMPLITUDES. 
The study of the seismograms reveals the periods of the vibrations in different parts of 
the disturbance as recorded at a number of stations. The actual amplitude of the earth 
movement can only be determined by a calculation which depends upon the period of the 
waves, the period of the pendulum, and the constants of the instruments. The calculation 
is made in accordance with the formulas, equations 79 or 81, page 169. In many cases the 
period of the pendulum is the same as that of the vibration and then, if there is not very 
strong damping, the magnifying power becomes indefinitely large and is undeterminable ; 
this is the condition at many stations where large movements are recorded by the seis- 
mographs; but it has been possible, in a number of cases, to determine roughly the true 
movement of the ground. The amplitudes on the two components at right angles to each 
other do not always reach their maxima at the same time; it sometimes happens that the 
movement is alternately strong on one component and the other. This was the case 
at Porto Rico, Upsala, and Pavia. Even when the maxima occur on the two components 
at the same time, one does not always get the true amplitude of the earth’s motion by 
taking the square root of the sum of the squares of the two components, for the difference 
of phase has an influence; but this is the only method we can use, and the quantities 
given in the column headed “ Poss. total” in table 19 were obtained in this way. 
DURING THE PRELIMINARY TREMORS, 
The vibrations during the first preliminary tremors frequently have periods in the 
neighborhood of 5 seconds; other periods were also present, but were not so persistent. 
At Jurjew the period during the first preliminary tremors was 29 to 30 seconds. It seems 
as tho there were many periods present and that the period which was close to that of the 
instrument was singled out and made prominent. At Sitka a period of about 17 seconds 
was shown. 
During the second preliminary tremors vibrations of various periods were also present, 
but those which had the largest amplitude seem to be about 15, 20, and 28 seconds. 
TaBLE 17.— Periods and Amplitudes during the Preliminary Tremors, 
First PRELIMINARY SECOND PRELIMINARY 
TREMORS. TREMORS. 
DIRECTION OF 
APPROACH. 

STATION. DISTANCE. CoMPONENT. 
in pe Period. rel Period. 

bo km. mm. sec. mm, sec. 
Sitka . . .| 20.72 | 2302 - : . «| 0.48 17 0.215? 15 
0.004 me x 
Ottawa . .| 35.387] 3930 0.005 
Washington .| 35.44 | 3937 : : Sigisct | ied Be 0.21 28 
Upsala . .| 76.80] 8533 ; . . .| 0.0013 0.37 19 
Osaka. . .| 77.30] 8589 : ; é eee | LOLOL Sac we 
Jurjew . .| 80.27| 8918 ics 

15 Sead aes 
Potsdam. .| 81.35] 9039 penned Hae et Pee 27 
Irkutsk . .| 80.82 orci 5 : 
Gottingen .| 81.36) 9040 
Leipzig . .| 82.40] 9155 
Jena . . .| 82.45}| 9161 
0.041 
Vertical . Q he 
North . . 0.028 
Hastie ae iin 
North. . 0.11 
North. ©. 0.0155 
Masten 0.0034 
North. rece 
sistem ee 
OLbDaeoune 0.006? 

Munich . .| 84.75 9417 
Vienna . .| 86.37 9596 
Batavia . .| 124.99 | 13887 





t Uncertain on account of closeness of periods. 
135 
