. Discovery. Science. 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 ty
. Discovery. Science. 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 " ; WTiere a station is fitted with apparatus for record- ing all three components, namelj-, nor
Size: 1579px × 1582px
Photo credit: © Paul Fearn / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcontribu, booksponsoruniversityoftoronto, booksubjectscience
