46 



MR. AIRY ON THE LAWS OF THE TIDES 



Comparing the coefficients in this table with the column of mean range of semi- 

 diurnal tide in page 35, we can discover no analogy between them. The range of 

 diurnal tide is not at all reduced in the North Channel, where the semidiurnal tide 

 is so much diminished ; nor (as will also be shown hereafter) is it particularly dimi- 

 nished between Kingstown and Dunmore East, where the semidiurnal tide is nearly 

 or quite obliterated ; but it is much diminished at Castle Townsend, where the semi- 

 diurnal tide is pretty large. 



On examining the interval from moon's transit, it appears evident that the diurnal 

 tide comes from the south, or very nearly from the south. It appears also that it 

 does not pass in either direction through the North Channel, but that the strait is 

 filled simultaneously, or nearly so, at both ends. It appears also that the wave travels 

 very quickly from south to north in the Irish Channel ; so quickly indeed that it is 

 probable that the tide is simultaneous throughout. But between Castle Townsend 

 and Passage West it loses more than six hours, or a quarter of a diurnal tide. I am 

 totally unable to explain this. The case is very greatly different from that discussed 

 in page 40, where the change of phase was almost exactly half a tide. I must leave 

 the solution of this difficulty to some more advanced theory of waves. 



I may appropriately close this section with a statement, in the form commonly 

 used by nautical persons, of the most prominent effects of the diurnal tide at the 

 several stations. 



Assuming that the maximum diurnal tide, with positive sign for the semidiurnal 

 high waters 1st division, occurred about July 9 and August 6, and with opposite sign 

 on July 23, it appears that the maximum takes place when the moon's right ascen- 

 sion is about 9''. This is not very accurate ; first, because the solar diurnal tide is 

 neglected ; secondly, because the days adopted are not purely for maximum at semi- 

 diurnal high water, but partly also refer to low water. Using however 9'', it appears 

 from the table in page 39, that the semidiurnal high water at Kilbaha follows the 

 moon's Greenwich transit by 4^' 47™ ; and, therefore, when the diurnal tide is great- 

 est, the semidiurnal tide at Kilbaha occurs at 1 S^My"* Greenwich sidereal time, or 

 - 13h 71' Kilbaha sidereal time. If this happens at noon, the sun's right ascension 



