AND CAH0RE SHINGLE BEACH, CO. WEXFORD. 31 



lines." The "nodal" or "hinge-lines" of the tide possess the fol- 

 lowing 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, 

 cceteris 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 Hull 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 con- 

 tinues, 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 Dun- 

 kirk. 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 "*. 



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 chan- 

 nels — 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 



* Beeckey, Phil. Trans. 1851. 



