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P-waves are longitudinal waves; that is, they travel in the same direction as the rock is moving back and forth. P-waves cause one part of the rock to compress together and another section to be pushed apart. This process is known as compression and rarefaction, similar to the propagation of sound waves.
The disc represents the full moon, traditionally the most auspicious time for work and celebration. The disc is set towards the hoist. Effective date: 1 January 1981.
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