THE RECENT ADVANCE IN SEISMOLOGY 403 
ment time of the earthquake at the distant observing station, and y, 
the duration of the first preliminary tremors reckoned in seconds. 
The time is thus obtained through subtracting from the time when the 
record begins the duration in seconds of the first preliminary tremors 
after multiplying by the factor 1.165. 
Careful analysis of earthquake records shows that the large waves 
may be further divided into four sections, designated the third, fourth, 
fifth, sixth, and sometimes additional sections of the seismogram 
(the two phases of the preliminary tremors being included in the 
numeration). The vibrations of the third section are few and slow, 
those of the fourth section are somewhat quicker and of very large 
amplitude, while those of section 5 are of much shorter period and of 
large amplitude. The durations of these different sections of the 
complete earthquake record are roughly equal to one another, the 
third and fourth sections being taken together. The amplitude is 
greatest in the fourth and fifth sections. The feeble vibrations which 
end the seismogram are called its “tail.” 
It has been rather generally held as a theoretic proposition that the 
direct waves which produce the preliminary tremors of the seismo- 
gram are longitudinal—that is, compressional—vibrations; whereas 
the “large” or “‘main” waves vibrate in the plane transverse to the 
line of propagation. A decisive experimental proof of the correct- 
ness of this view seems to have been happily furnished by the regis- 
tration of the recent Kingston earthquake of January 14, 1907, by 
the seismograph of the U. S. Weather Bureau.t’ The two Borsch- 
Omori pendulums of the bureau are so placed as to record the north- 
south and the east-west components of the wave motion. Now it 
happens that the port at Kingston, which is something more than 
1,400 miles distant from Washington, differs in longitude by only 15 
minutes. For our purposes, therefore, Kingston may be considered 
as located upon the meridian of Washington. Practically no pre- 
liminary tremors were registered in the east-west direction at the time 
of the earthquake, though a very distinct series was recorded in the 
north-south direction. Notwithstanding this difference, the main 
waves appeared at practically the same instant in the two records, but 
tC. F. Marvin, ‘The Kingston Earthquake,” Monthly Weather Review, January 
22 sTOO7 Pps lads 
