National Geographic : 1898 Nov
466 WHAT IS THE TIDE OF THE OPEN ATLANTIC? fluctuations of pressure by means of an electrical communica tion with the surface. All study of the tides must therefore proceed from the shores. SUBDIVISION OF AREA The tidal stations for our area fall naturally into two groups as regards distribution in (1) the land-locked waters of the shore itself and (2) the shallow waters bordering North America on the east. Brief notes on the tides of the first area (estuarine) have already been published in the September number of this magazine. Certain water bodies of form not unlike the estuaries there studied could not be included in that paper from the anomalous character of their tides. These are the Bay of Fundy, Vineyard sound, Buzzards bay, Narragansett bay,and Long Island sound. For these waters and the general tidal phenomena of the shallow offshore waters we get light from the consideration of the tides in the open Atlantic, and we immediately see that the older view of the ocean tides is in conflict with the facts now widely observed. This was the view of the progressive wave and the cotidal lines. Many difficulties are smoothed over by limiting this conception to the shallower shore waters and sup posing the ocean basin to be the seat of a stationary wave with vibration period adjusted to the motion of the moon. PROGRESSIVE AND STATIONARY WAVES A pebble dropped into still water sends circling ripples in every direction from the point of plunge. The ripple is a little wave that travels off till overcome by frictional resistances or stopped by the shore. It is a progressive wave. To form it a number of water particles in succession move up, forward, down, and back, as may be noted by floating sticks and straws. Such a wave is produced at or off the mouths of estuaries and travels up them. The velocity is supposed to be that acquired by a body falling freely through one-half the depth of water.* If you lift one side of a basin or tub partly filled with water and quickly lower it again, the water within oscillates as a whole in a time dependent for any one vessel on the depth of water. The water on opposite sides rises and falls, up at one side when down on the other. Along a line across the center there is no *To make this available in rivers we need a formula for integrating the varying depth and recognition of the effect of width. Now that the Delaware has been gauged, such a study is possible.