2
TIDAL AND CURRENT DATA
(F. and C.) are therefore the index periods of tidal movements. The tides of new moon average a few inches higher than those of full moon.
   The Tidal Wave.— The tidal impulse originates in the Southern Ocean, its impetus travelling thence into the Atlantic and Pacific Oceans. The height of the tidal wave traversing mid-ocean probably does not much exceed a maximum of 2 feet. Its pace is 350 to 400 miles per hour, equivalent to a race from the Cape of Good Hope to the Forth in twenty-four hours. From the cotidal lines, i.e. the lines laid on a chart connecting places where the times of high water coincide, the route of the tidal wave may be defined. Its local irregularities are in many localities obscure. This perennial fund of extra-terrestrial energy was called by Scott-Russell the great primary tidal wave, and it is the source of a tide movement which affects the whole mass of ocean waters. Considering that depths of water of 3 or 4 miles exist in its course, the tidal pulse may be compared to the heart beat of the world. Impinging on contiguous coastlines its diurnal travel causes an infinite variety of effect.
   The solar heat playing on equatorial regions of the ocean at the same time sets vast currents in motion. The combined movements of the tidal wave and of oceanic currents striking headlands or trapped in embayments result in a maze of local sea disturbance, and it is the function of the harbour and foreshore engineer to control or harness these forces to his purpose.
   Standards.—The official gauge by which tidal movement is recorded in Great Britain is that of Ordnance Datum (O.D.). This datum is the standard to which variations of level are referred on the Government Survey Department maps. Harbour Boards, however, pretty generally employ independent local standard gauges. Thus the Port of London Authority adopts “Trinity High Water” (T.H.W.) as its standard. This represents mean high-water level at London Bridge, and is 12 feet 6 inches above O.D. Ordnance Datum is in effect a mark on a brass gauge at Liverpool, recording the mean tidal level at that port. The relative standards of other ports above or below this level are ascertained by the average of a long series of observations (fig. 1).