This plate illustrates three quite different subjects. The two larger upper maps on this plate show geomagnetism. Since the North Magnetic Pole is in a different position from the Geographical North Pole, and since the lines of the earth’s magnetic forces are deflected by various agencies, the compass needle does not point toward the Geographical North Pole in most locations. The deflection of the compass needle from True North is called magnetic variation or declination. Thus, for example, where variation is west, north as indicated by the compass needle is west of True North by the number of degrees marked on the isogones, as illustrated by the upper left map on this plate. The upper right map indicates the average annual change of magnetic variation in minutes. The small inset map at the top of this plate, entitled Earthquake Probability, shows the damage which may result from earthquakes occurring in different parts of the country. The zones are graded from 0, where such earthquakes are likely to do no damage, to 3, where earthquakes are likely to do major damage. The small-type numbers indicate the magnitude of some recorded earthquakes. The index of magnitude is related to energy released rather than the damage done. A magnitude 5.6 is considered as the threshold at which damage begins. The four lower maps on this plate show co-tidal and co-range lines for/from the semi-diurnal and diurnal tides. A co-tidal line indicates the position of the crest of the tidal undulation at a given time. The hours marked on these lines are intervals in lunar time from that instant when the moon crosses the Greenwich meridian. A co-range line indicates the difference in level between the crest and the trough of the undulation.
