ON THE COASTS OF IREJiAND. 29 



the days of evanescence for semidiurnal high water and for semidiurnal low water is 

 small, and therefore it appears that in the case of the Irish tides in question, the 

 difficulty of separating the solar and lunar effects is inevitable. 



A treatment of the phenomena of diurnal tide which does not lead to a distinction 

 of the solar and lunar effects must be considered as imperfect. And it has been ex- 

 plained that, though from a short series of observations the distinction cannot be 

 extracted, it can with certainty be obtained from a long series of observations. 

 With a full avowal of the completeness of my own failure and with a statement at 

 the same time of what science requires and what it may reasonably expect, I may be 

 permitted to explain an expression which I have used in my tract on Tides and 

 Waves, Art. 564, where I have said that a determination by Mr. Whewell is worth 

 little. My intention was to express that the distinction of the solar and lunar effects 

 was theoretically important, that it might be obtained from the observations there 

 referred to, and that it was not obtained. In stating that Mr. Whewell s determi- 

 nation was worth little, my expression was thus far incorrect, that a general rule for 

 the order of diurnal tides, though liable to some inaccuracy, was really obtained by 

 Mr. Whewell. I regret that I should have made a statement which could thus 

 seem to be insufficiently founded, and still more that 1 should have expressed it in a 

 phrase which could be interpreted as lightly esteeming the deductions of the writer 

 to whom we are indebted more than to any other for the knowledge which we pos- 

 sess regarding the laws of the tides. 



Section V. — Discussion of the height of apparent mean water, as deduced from the 

 heights of high and low water only, corrected for diurnal tide ; with reference to 

 difference of station, and to variations of the magnitude of the tide and of the moon\s 

 declination. 



The heights at high and low water were corrected for the diurnal tide in height 

 found in Section III. The age of the semidiurnal tide as deduced from heights 

 having been found (by the process of the next section) to be about two days, the 

 times were taken from the Nautical Almanac, at which the moon s hour-angle from 

 the sun was 3^, 9\ 15^, 21^, and two days being added to these, the times were de- 

 termined which were to be used as separating the large tides from the small tides. 

 The groups of observations thus marked off were the following : — 



d h d h 



1st Group, June 28 6 to July 6 16, small tide. 



2nd Group, July 6 16 to July 13 3, large tide. 



3rd Group, July 13 3 to July 20 9, small tide. 



4th Group, July 20 9 to July 28 9, large tide. 



5th Group, July 28 9 to August 5 0, small tide. 



6th Group, August 5 to August 11 16, large tide. 



7th Group, August U 16 to August 18 15, small tide. 



