1920.] Hogben.—Interpretation of a Typical Seismogram. 107 
feature : very few seismograms show more than two or three phases clearly 
enough for us to get the exact time of the beginning of each phase ; but 
this shows clearly six phases, marked'—(1) P, (2) PR 1? (3) S, (4) PS, (5) SR 1? 
(7) \j t ; and four others less clearly—(6) x, (8) L 2 , (9) L 3 , (10) L 4 . 
The following explanation of the waves (1) to (5) and (7) is now 
generally accepted by seismologists. The character of the waves x, L 2> 
L 3 , and L 4 is less certain. 
(1) The P waves (undue frimae) are considered to be direct longitudinal 
waves propagated through the mass of the earth along the path of shortest 
time (brachistochronic path), which is concave to the earth’s surface ; 
similarly (3), the S waves (undae secundae), are direct transverse waves ; 
the paths of P and S from the origin 0 to the place of observation A, 
although not identical, may be indicated roughly by OCA ; their times of 
transit are 871 seconds and 1,595 seconds, according to the tables (see 
below), and their apparent mean velocities are 13-3 kilometres and 7-3 kilo¬ 
metres per second respectively. (The distance from the epicentre (A), 
104°-25, or 11,583 kilometres, corresponds to a value of 724 seconds for 
S-P.) 
B 
(2) PR 4 are P waves reflected once at B, the middle point of the 
arc OA ; they will therefore take just twice as long to reach A as P waves 
will to reach B. 
(5) SR 4 are S waves reflected once at B. The paths of PR 4 and SR x 
(though not identical) are represented approximately by ODBEA. 
(4) The PS waves : When vibratory waves fall upon the inner side of 
the earth’s surface they are are not all reflected in accordance with the 
law that the angle of reflexion = the angle of incidence, but some undergo 
diffraction, and are reflected abnormally, as OF, FA, or 00, OA. When 
the angles of the reflected waves with the normal differ greatly from the 
angles of incidence, a series of waves originally longitudinal (e.g., along OF) 
will be transmitted along the rest of the path (FA) as transverse waves 
and waves that began as transverse waves (along 00) will complete 
their journey (along OA) as longitudinal waves. When the total time 
is a maximum the effect will be most marked ; here OF = OA == 23°, 
FA = 00 = 81J° ; obviously the two series will arrive at A at the same 
instant, and will reinforce one another,- thus making the effect still more 
marked. 
Lj are the long waves (undae longae) propagated as surface waves along 
the arc OA with a velocity, according to Dr. Klotz, of 3-8 kilometres per 
second. 
During the last thirty years a great many world earthquakes have been 
instrumentally recorded, and seismologists in various parts of the world 
have attempted to draw time-curves expressing the time taken to travel 
