. 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
. 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
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