SIDEREAL MOVEMENTS OR TIDES In Art. 2 we defined the term " sea-level " as the general level of the surface of the ocean, irrespective of the essentially surface-disturbances produced by winds, or the periodical rising and falling of the water known as the tide. Although wind-waves are occasionally of considerable height, yet they are local and temporary, affecting the surface only, and subside when the wind falls. But the tide is a movement that affects the whole mass of the water, which flows and ebbs every twelve hours. The tidal-wave is lowest in the uninter rupted open sea, and highest in more limited areas, especially in bays, gulfs, or channels opening broadly to its course, and gradually decreasing in width, such as the English and Bristol Channels. The height of the tidal-wave also varies consider
ably at different times, as well as in different places ; but what ever the height of the wave, the elevation or flow above the general level is always equal to the depression or ebb below that level As a standard of measurement for elevations on the land, or the depths of the sea, neither the high-water nor the low-water mark can be taken, but the mean between the two, which is always the same, whatever the variation in the height of the tide at different times and in different places. Thus the datum line of the Ordnance Survey of Great Britain is fixed at the mean tide-level at Liverpool