Page:Quarterly Journal of the Geological Society of London, vol. 33.djvu/57

lines." The "nodal" or "hinge-lines" of the tide possess the following properties:—

First. The least rise and fall of the tide takes place along these lines.

Second. The quantity of water which passes during the flow and ebb of the tidal stream through a section drawn along these nodal lines is greater than the quantity of water passing through any other section of the seas; and consequently, cæteris paribus, the tidal stream is greater along these lines than in any other section of the channel.

As the tidal currents flow and ebb to and from the "head of the tide," there must be two nodal lines, one on each side of the "head of the tide;" and such we find in the North Sea and English Channel and in the Irish Sea. In the North Sea it is situated near Yarmouth, on the Norfolk coast, and in the English Channel at Swanage Bay, Dorsetshire; while of those in the Irish Sea, the northern one occurs between Ballycastle aud the Mull of Cantyre, and the southern one between Courtown and Aberystwith. With the nodal lines and their accompanying currents at Swanage Bay and Courtown we are most interested, as they respectively affect the Chesil and Cahore beaches.

In the Irish Sea the "head of the tide" is a line across the greatest breadth of the sea, while the "head of the tide" between the North Sea and the English Channel is a line across the Straits of Dover, the narrowest section of the sea. This dissimilarity gives rise to some remarkable differences as to the meeting of the tides in the two cases. In the Irish Sea, west of the Isle of Man there is a tract apparently tideless, and the line of the "head of the tide" is constant (Full and Change); but in the Straits of Dover the line of meeting and of separation oscillates during each tide between Beachy Head and the North Foreland (60 miles) in the following manner:—"When the water on the shore at Dover begins to fall, a separation of the Channel stream takes place at Beachy Head; as the fall continues, this line of separation creeps to the eastward; at two hours after Dover high water it has reached Hastings; at three hours, Rye; and thus it travels on until at low water, by the shore, it has arrived nearly at the line joining the North Foreland with Dunkirk. At this time the Channel stream on both sides is slack, but for the 60 miles from the Foreland to Beachy Head is still running to the westward. When the water begins again to make on shore, the Channel streams commence to run towards the strait from both sides, and the line of separation again occurs at Beachy Head, and begins to travel again slowly to the eastward "^[1].

The nodal lines are also lines of "tide and half-tide;" for when it is low water at the "head of the tide" in the Irish Sea, the stream commences to flow both in the north and south channels—that is, three hours before high water at the nodal lines; so that the "offing tide" begins to flow at half-flood on shore; and in

1. Beechey, Phil. Trans. 1851.