128 REPORT OF THE CALIFORNIA EARTHQUAKE COMMISSION, 
by the major arc; or we may find the interval between the arrivals of the waves by the 
two routes by multiplying the second quantity by 2 ¢. This process can be carried out 
graphically with ease for seismograms having a small time scale, such as those of Milne 
pendulums. Mark on the seismogram (fig. 30) the point 0, the moment when the earth- 
quake occurred at the focus; at any point, as for instance p’, erect a perpendicular 
° —- 
p’e’, equal in length to op’ a“ ; draw a straight line oe’, and produce it; the 
height pe of this line above any point p of the seismogram will represent half the inter- 
val after p, before the arrival of the surface waves by the major arc, corresponding to 
those which, following the minor arc, are recorded at p. If we cut from a sheet of paper 




Fig. 30. 
KK, a triangle abc, such that ac equals 2 ab, and place the triangle so that ac lies along the 
medial line of the seismogram, the point c will mark the place where the major arc waves, 
corresponding to the minor are waves recorded immediately under the point where be cuts 
oe’, will be recorded; by this device the whole seismogram can be examined in a few 
minutes. This method must be modified to apply to seismograms with open time scales, 
and it then requires a very large space; it is simpler, with such seismograms, to calculate 
T directly, with a slide-rule, from the first expression given above. When we apply this 
graphical method to the Milne seismograms we find, in the majority of cases, that there 
are marked swellings on the seismogram at the time the waves of the strong motion would 
arrive by the major arc. The seismograms of instruments with open time scales yield 
much less definite results; indeed, in the majority there is no sufficiently well-marked 
increase in amplitude to make one certain that the major arc waves have produced any 
sensible effect. 
The seismograms yield various results, as follows : 
Honolulu. —The swellings from 16" 00™ to 16" 18™ mark waves arriving by the 
major are corresponding to the strongest motion of the direct minor arc waves. If the 
strong motion recorded at 14" 30™ is due to surface waves, the corresponding major are 
waves would appear at 24", long after the record was over. A small disturbance lasting 
for an hour is reported about 45 minutes after this time. If the movement mentioned is 
really due to surface waves arriving by the minor arc, their velocity of propagation would 
be about 1 km./sec., which is so extremely slow that weare led to discard this explanation 
of its origin. 
