THE SEISMOGRAM AND ITS ELONGATION. 111 
reflections and refractions, prolong the time during which this group reaches the surface. 
In this group, as in the first, the proportion of longitudinal and transverse vibrations 
reaching the surface may vary between wide limits; but the longitudinal waves can 
never predominate. However, the first vibrations of the group will be longitudinal; 
for at the first refracting surface which the waves meet longitudinal waves will, in general, 
be generated, and will immediately advance at a higher speed, always keeping ahead of 
any transverse waves that they may develop. ‘These waves, like the leaders of the first 
group, are apt to be weakened by reflections and transformations and may fail of recog- 
nition when they are superposed on the later vibrations of the first group. The time of 
arrival of the first group is dependent on the speed of the longitudinal waves, from start 
to finish; but that of the second group depends on the speed of transverse waves in the 
homogeneous interior and of longitudinal waves in the heterogeneous outer layer. 
We do not know enough about the interior of the earth to fix the thickness of the outer 
heterogeneous layer, nor to say whether severe earthquakes originate in it or below it; 
tho the former seems the more probable. We have for simplicity of statement assumed 
the latter, but this is by no means necessary. If the earthquake originated in the hetero- 
geneous layer, both groups of waves would suffer some elongation before they reached 
the homogeneous interior and after they left it; but they would travel without change 
so long as they were init. If there is a central metallic core in the earth, changes would, 
of course, take place when the waves crost its boundary. 
THE STRONGER TRANSVERSE WAVES. 
If the outer layer of the earth were sufficiently thick or sufficiently heterogeneous, longi- 
tudinal and transverse vibrations of the preliminary tremors might become so mixed in it 
that the first and second phases would not be distinguishable; but nevertheless, the two 
kinds of waves would separate from each other in the homogeneous interior and at distant 
stations the two phases would appear. But the fact that the second phase is so much 
stronger than the first at all stations, including those 30° or 40° from the origin, which are 
too near for the difference to be accounted for by the vertical component of the longi- 
tudinal motion, indicates that the outer homogeneous layer by no means destroys the 
distinction between longitudinal and transverse waves in the first two phases; and that 
the transverse waves are originally much stronger than the longitudinal. This may be 
due to the way in which the waves originate at the fault-surface. When the rupture 
occurs there, the friction of one side against the other is probably the chief means of 
starting the vibrations, and evidently would produce stronger transverse than longitudinal 
waves. 
THE SEPARATION OF THE FIRST TWO PHASES. 
The distinction of the first two phases would exist from the very start, but they would 
naturally reach a near station only a few seconds apart; and if the original shock lasted 
longer than this interval and underwent considerable variations in intensity, the arrival 
of the first preliminary tremors, due to successive parts of the shock, might mask the 
arrival of the second. Moreover, and this fact is perhaps still more important, few in- 
struments are provided with very open time-scales, a necessary condition to show the 
separation of the phases near the origin. Fortunately the Ewing three-component seis- 
mograph at Mount Hamilton met this requirement; its time-scale was 6 or 7 mm. to the 
second and it was therefore quite competent to show the interval of 9 seconds which 
separated the beginnings of the first two phases. Mount Hamilton, at a distance of 128 
km. from the origin, was the nearest station provided with a time-marking record. At 
Victoria (distant 10.41° or 1,156 km.) the smallness of the time-scale and the overlapping 
