Diurnal Tides on the Coasts of Ireland. 43 



rent High and Low Water. The remaining six, three belonging to the Solar, and 

 three to the Lunar Diurnal Tide, are to be determined ; and being found, the 

 values of Z), calculated from (2), are to be compared with its values deduced 

 from observation by means of equation (1). 



The unknown quantities of the Diurnal Tide are, therefore,— 



1st. The coefEcients of Solar and Lunar Tide. 



2nd. The Diurnal solitidal and lunitidal Intervals. 



3rd. The Ages of the Solar and Lunar Tides. 



Of these quantities, one, viz., the Age of the Solar Diurnal Tide, cannot be 

 found from observation, because the Sun's place, or declination,changes so slowly 

 that it is a matter of indifference what place we assign to the Sun (within a limit 

 of some days) in estimating the amount of the Solar Tide. The other five 

 quantities may and have been found from the observations, as I shall presently 

 show. 



The constants of equation (2) were found as follows for each of the ten 

 Tidal Stations. An inspection of equation (2) shows that the Solar Diurnal 

 Tide disappears at the equinoxes (because 5 = 0, or is very small) ; hence, the 

 equinoctial Diurnal Tide, observed at High and Low Water, was altogether due 

 to the Moon, and the Lunar Diurnal Tide was found approximately from the 

 equinoctial tides, and constructed on the same abscissa as the observed Diurnal 

 Tide. This Lunar Tide constructed from calculation differs considerably from 

 the observed Diurnal Tide at the solstices, the difference being due to the Solar 

 Diurnal Tide ; in this way the Solar Diurnal Tide was in its turn calculated ap- 

 proximately from the solstitial tides, and the calculated Solar Tide carefully su- 

 perposed upon the Lunar Tide. 



The observed and calculated tides, constructed as just described, were then 

 compared both with reference to the maximum heights at High and Low Water, 

 both positive and negative, and with reference to the times of vanishing of the 

 Diurnal Tide at High and Low Water ; and from this comparison the constants 

 used m the construction were corrected, and the heights and times again com- 

 pared, until the agreement was as close as the observations would allow. 



The constants thus successively corrected are given at the end of the 

 Table of Diurnal Tides, for each station, and the comparison of the observed 



g2 



