When sea waves have travelled long distances from their origin, as, for instance, whenever a South American wave crosses the Pacific to Japan, the phenomena which are observed are like those which were observed at Acapulco; the sea falls and rises, at intervals of from ten minutes to half an hour, to heights of from six to ten feet, without the slightest appearance of a wave. Its phenomenon is like that of an unusually high tide, which repeats itself several times per hour. Even if we watch distant rocks with a telescope, although the surface of the ocean may be as smooth as the surface of a mirror, there is not the slightest visible evidence of what is popularly called a wave. The sea being once set in motion it continues to move as waves of oscillation for a considerable time. In 1877, as observed in Japan, the motion continued for nearly a whole day. The period and amplitude of the rise and fall were variable, usually it quickly reached a maximum, and then died out gradually. As observed in a self-recording tide gauge at San Francisco, the disturbance lasted for about four days. A diagram of this is here given. In its general appearance it is very similar to the records of other earthquake waves. The large waves represent the usual six hours rise and fall of the tides; usually these are fairly smooth curves. Superimposed on the large waves are the smaller zigzag curves of the earthquake disturbance, lasting with greater or less intensity for several days. As these curves are drawn to scale—horizontally for hours, and vertically one fifth inch to the foot, to show the extent of the rise and fall—they will be easily understood. Sometimes, as in the present example, the first movement in the waters is that of an incoming wave. In many instances, however, this observation may be due to the slow and more gentle phenomena of the previous drawing out of the water, which in a steep waste, or when the water is rough, would be difficult to observe, not having been remarked. The distance to which these sea waves have extended has usually been exceedingly great. [Illustration: FIG. 28.—Record of Tide Gauge at Port Point, San Francisco. Showing Earthquake Waves of May 1877.]