38 



MR. AIRY ON THE LAWS OF THE TIDES 

 Age of Tide as inferred from Heights. 



In the Encyclopaedia Metropolitana, Tides and Waves, Art. 545, I have given 

 reasons for believing that the true age of the tide is that given by the heights. From 

 the inspection of this table it appears certain that the age of the tide on the western 

 coast of Ireland is almost exactly two days. The different results are upon the whole 

 very consistent. Yet there are some discordances which I cannot entirely explain. 

 The most distinctly marked discordance is this — that the age given by the Low 

 Waters is, in every instance except one, greater than that given by the High Waters. 

 I have shown, I think, with certainty, in the Encyclopaedia Metropolitana, Tides and 

 Waves, Articles 452 and 544, that the cause of the principal part of the apparent age 

 of tide is friction. It appears to me not unlikely, that, on carrying out the theory of 

 friction in combination with the consideration of oscillations bearing a finite propor- 

 tion to the depth of the sea, the apparent age of the tide might be found greater for 

 low waters than for high waters. But I have not examined the theory so far as to 

 feel myself warranted in positively assigning this as the explanation. 



Section VII. — Establishment of each por^t, and progress of semidiurnal tide round the 



island. 

 The process used in this investigation is very nearly similar to that by which the 

 table in page 30 was formed. The mean interval from the moon's transit at Green- 

 wich to the time of tide was found (by tiie operations of the next section) to coincide 

 with the actual interval nearly at the following times : — Port Rush and Ballycastle 

 (high and low), June, 22^ 9^ 30^ 20^^ ; July, 'J^ W\ 14^ 21'', 21'^ 23^, 30*^ 16*' ; August, 

 6'^ 4^ 13'' 5^ 20'' le'' : Kilbaha, Kilrush, Casleh Bay, Galway, Mullaghmore, Buncrana, 

 Carrowkeel, Glenarm, Donaghadee, Ardglass, Clogher Head, Kingstown, Castle 

 Townsend, (high and low), 1*^8'' later; Foynes Island, Okl Head, Dunmore East, 

 Passage West (high and low), Limerick (high only), 20^ later than the last ; New 

 Ross (high and low), 23'' later than the last ; Limerick (low only), 22*' later than the 

 last, or 4* 1* later than at Port Rush and Ballycastle. The intervals from the moon's 

 transit to the time of tide being taken for every tide (the high water, 1st division, 

 being referred to the upper transit, the low water, 1st division, to the transit over the 



