THE PROPAGATION OF THE DISTURBANCE. 125 
There is one group of reflected transverse waves which have especial interest, namely, 
those whose angle of incidence is so large that they experience innumerable reflections, 
that is, they practically creep around the earth’s surface. Professor Knott has sug- 
gested that they are the so-called surface waves.’ But there are certain obvious objections 
to this idea. They are, that the speed of propagation could not be less than the speed 
of the transverse waves near the surface of the earth; this speed appears to be about 
4.8 km./sec., considerably greater than that of the long waves; again, the energy in the 
surface waves is much greater than in the second preliminary tremors, but possibly the 
distribution of energy on account of the change of velocity, with the depth below the sur- 
face and the retention of energy by the transverse waves creeping along under the 
surface, may account for this; lastly, observations do not show consistently that the 
surface waves are made up in large proportion of transverse waves (see page 114). But, 
nevertheless, Professor Knott’s suggestion is a very interesting one, and it is quite pos- 
sible that these objections may be overcome when we have more accurate knowledge of 
the various quantities concerned. 
It is difficult to find the time of arrival of waves reflected once in the major are. The 
minor are must be greater than 120° for half the major are to be less than this value, 
which is the limit to which the hodograph can be relied upon. The first preliminary 
tremors, after reflection in the major arc, are apparently too weak to be evident on the 
seismogram. It would take about 55 minutes for the transverse waves, reflected once 
on the major arc, to reach stations beyond 120° from the origin; that is, they would reach 
them at about 14°07"; at Bombay the motion becomes most irregular at this time; at 
Batavia there are variations of intensity, but nothing very definite; at Kodaikanal 
and at Perth the seismograms are stronger at about this time. Altho we can not give 
the exact time at which reflections on the major are would reach stations at a less dis- 
tance than 120° from the origin, the hodographs show that they could not possibly be 
earlier than the arrival of the regular waves, and, therefore, they are completely masked 
by the much stronger disturbance existing during the regular waves, the principal part, 
and the earlier parts of the tail. 
THE SURFACE WAVES. 
In addition to the times of arrival of the first two phases we have plotted in plate 2 
the times of arrival of the regular waves. The surface waves are spread over many min- 
utes on the seismogram, but as already noted, we have taken as the beginning of the 
regular waves that point where the irregular movement (which is a part of, or follows, the 
second preliminary tremors) becomes regular, with a long period (80 to 50 seconds). 
The plotted positions of these times of arrival lie very closely along a straight line and no 
other simple curve could be drawn which would fit the observations materially better. 
To determine the best straight line to use we resort to the method of least squares, but 
as the observations differ very much in their reliability, each one is given a suitable weight. 
No elaborate distribution of the weights has been made. ‘The observations which are 
considered good have received the weight 5, those which are fair 3, and those which are 
doubtful 1; a few observations which are very doubtful have been left out altogether. 
Where several stations have been grouped together the weight of the average is, of course, 
the sum of the weights of the individual stations. Those observations are considered 
doubtful which, on account of the absence of the seismogram, could not be checkt, or 
in which the seismogram does not show clearly just where the regular waves begin. In 
table 14 we have collected the observations which have been used in determining the 
straight hodograph of the regular waves and their weights. 


1The Physics of Earthquake Phenomena, p. 256. 
