142 REPORT OF THE CALIFORNIA EARTHQUAKE COMMISSION, 
Time scale. — A great variety of time scales were used, from 1 mm. to the minute, or 
even less, up to 15 mm. to the minute; and in one case, at Gottingen, the scale was 60 mm. 
to the minute. The advantage of the open time scale is that individual vibrations are 
recorded, making it possible to determine their period and the magnifying power of the 
instrument for them; and the characteristics of the motion can be seen. This can not 
be done on seismograms with small time scales. On the other hand, when the movement 
begins very gently it is extremely difficult, on the open time scale, to determine where the 
slight waves in the line begin; but they would appear much more clearly on seismograms 
with small time scales. Wherever the magnifying power is sufficiently great there is no 
difficulty in determining the time of the beginning, and the advantage of a time scale of 
10 or 15 mm. to the minute, in permitting the period of the vibration to be determined, 
is very great. 
Identification of the phases on the seismograms. — The seismograms made by different 
instruments differ greatly among themselves, and it is very often extremely difficult to 
' decide exactly where a particular phase begins. Where the magnifying power is suffi- 
ciently large this difficulty is not serious for the first and second preliminary tremors, 
but it often is for the regular waves and the subsequent phases. Where the magnifying 
power is small there is great difficulty in deciding upon the time of the beginning of the 
first preliminary tremors. It is therefore of very great importance, in studying the 
propagation of an earthquake disturbance, to have copies of the seismograms themselves, 
and not merely the recorded times as determined by the directors in charge of the 
instruments. For without doubt, different persons examining single seismograms, with- 
out comparison with others, would frequently take different parts of the movement to 
represent the beginning of a particular phase. 
