ON THE COASTS OF HiELAND. 



39 



meridian 6^ west, the high water, 2nd division, to the lower transit, and tlie low water, 

 2nd division, to the transit over the meridian 18^ west), groups were formed divided 

 i)y the times above mentioned, and the mean of the intervals was taken for each group. 

 Thus the following Table was formed : — 



On inspecting the intervals for Castle Townsend, Kilbaha, Casleh Bay, Galway, 

 Old Head, and MuUaghmore, it appears evident that the semidiurnal tide approaches 

 the western coast of Ireland very nearly from the west, or possibly from a direction 

 a little south of the west. On examining the intervals at Port Rush, Ballycastle, 

 Glenarm, Donaghadee, their regular progression seems at first to show that the tide 

 wave enters the Irish Sea by way of the North Channel. But the objections to this 

 supposition are most serious. Theoretically it is certain that (even without regarding 

 the effects of friction) when a tide-wave passing through a narrow channel enters into 

 a wide one its vertical range will be diminished*. But here the mean range in the 

 narrow channel at Ballycastle is only 2J feet and at Glenarm 5^ feet : this tide-wave 

 therefore could not possibly produce a tide of more than one or two feet in the Irish 

 Sea ; whereas, from Ardglass southward to Kingstown, the range is from ten to twelve 

 feet. It seems therefore that this supposition may at once be dismissed ; and the 

 only supposition which can be substituted for it is, that the tide-wave enters the Irish 

 Sea by St. George's Channel. But here a most remarkable circumstance occurs. A 

 reference to the table above will show that the high water at Kingstown occurs six 

 hours and a few minutes later than that at Dunmore East, or, in other words, that 



* Encyclopaedia Metropolitana, Tides and Waves, Art. 264. 



