DISCOVERY 



313 



Let us take an illustration to make this point clear. 

 If two trains leave London for the North, travelling 

 at 60 and 30 miles per hour respectively, the express 

 will reach a point 60 miles distant in 60 minutes, 

 whereas the slow train will pass the same point in 

 2 hours, just 60 minutes later than the express. It is 

 seen that the slow train is getting late at the rate of 

 one minute for everj^ mile run, hence at anj? point in 

 its journey its lateness in minutes equals its distance 

 in miles from London. 



So with the earthquake ; the velocities of the various 

 types of wave being known, as a result of manj' 

 observations on previous shocks, the difference in 

 the time of arrival of any pair is a measure of the 

 distance of the origin, their common starting-place. 

 For instance, when the secondary waves arrive 

 9 minutes 25 seconds later than the primary, the dis- 

 tance of the earthquake from the observatory is 5,000 

 miles ; and it can be further computed that the shock 

 occurred 11 minutes 35 seconds earlier than the time 

 of recording the first primary wave. 



Before leaving this question of rates of propagation, 

 it is interesting to remark that the first impulse arrives 

 at the antipodes in about twenty-one minutes, after 

 which, in the case of severe shocks, the whole Earth 

 continues to pulsate for four to ii\'e hours ; and 

 though it is far too feeble to be felt, every particle of 

 matter and every living soul are rocked to and fro 

 during this time. The titanic force of an earthquake 

 is too stupendous for our full comprehension. 



Locating the Position 



The first primary wa\'e which arrives at a station 

 indicates the direction. It is either a " push " or a 

 " pull" in a direct line with the centre of the shock. 

 If this wave is accompanied with a rise of the ground, 

 it is a " push " ; if the ground faUs, it is a " puU." 



WTiere a station is fitted with apparatus for record- 

 ing all three components, namelj-, north-south, east- 

 west, and the vertical motion, and a good record is 

 obtained, it is possible to determine the direction from 

 the data of one observatorv. 



A more satisfactory method is that where three 

 or more observatories compare their readings. Each 

 station calculates its distance from the epicentre. 

 With three such distances as radii, and the respective 

 stations for centres, circles are drawn upon the globe. 

 At one point aU three circles wiU meet. It will be at 

 that point where the earthquake has occurred. 



More precise determinations are made by the Uni- 

 versity Observatory at Oxford. This station, under 

 the supervision of Professor H. H. Turner, F.R.S., is 

 the international centre for collecting the reports from 

 the various observatories throughout the world. 



Earthquakes occur chiefly along well-defined belts 



on the Earth's surface. One belt nearly encircles the 

 Pacific, the other stretches in a fairly straight line 

 from the \\'est Indies through the Atlantic Ocean, 

 Southern Europe, and Asia, terminating in a fork 

 nmning into Japan and the East Indies. 



When the distance has been calculated for any 

 station, a circle of that radius may be described around 

 it. It is then possible to note at which points the 

 circle cuts the main belts, thus narrowing down the 

 possibilities where the earthquake may be. This 

 method lacks very considerably in acmracv, biit in 



the absence of other data it has the merit of being 

 expeditious. 



The Seismograph 



Let us now consider the apparatus whereby the 

 record, or seismogram, is made. 



The basis of the seismograph in most cases is a 

 pendulum whose movements, relative to the surface 

 of the ground, are highly magnified before recording 

 them. 



The usual clock pendulum swings vertically, fairly 

 quickly, and is not sufirciently sensitive for the work. 

 It has been found that when a pendulum is mounted 

 horizontal!}' as shown in Fig. 2, where the top point 

 of support is nearly vertically' above, but a little for- 

 ward of the lower point on the rod, it beats much 

 more slowly, and becomes considerably more sensi- 

 tive either to a horizontal thrust or tilting of the 

 ground. 



If the Earth movement approaches the pendulum 

 end on, the pendulum is not affected; therefore to 



