ON THE COASTS OF IRELAND. 21 



Upon comparing them it appears that there is a very good agreement of the numbers 

 of the littoral stations, at both high and low waters, as far as Mullaghmore or even 

 Buncrana; and, for high water only, as far as Ballycastle. There is then great dis- 

 cordance till we arrive nearly at Kingstown; in a short time after this the diurnal 

 tide becomes so small that we are less surprised at apparent discordances. From 

 the number of the instances in which the agreement is, upon the whole, pretty good, 

 I form my opinion that the discordance between Buncrana and Kingstown is not 

 accidental. I have little doubt that in this channel between Ireland and Scotland 

 (which every accurate determination shows to be a critical part for the tides), the 

 law of diurnal tide assumes a form differing much from that supposed in the investi- 

 gation. It is, however, practically almost impossible to trace this law from obser- 

 vations. 



The results for diurnal tide used in the subsequent investigations are those in the 

 table of pages 15 and 16 deduced from observed heights only. 



Section IV. — Theory of Diurnal Tide as referred to the actions of the Sun and Moon. 



The present section will contain little more than the account of a series of failures 

 of investigations. But the examination of these is usually so instructive that I think 

 it desirable to state the heads of each of the unsuccessful attempts. 



In order to explain the theoretical difficulties of this investigation, the following 

 remarks may not perhaps be misplaced. 



It is not possible to separate the effects of the sun and moon by comparison of a 

 mass of observed diurnal tides near one solstice with a similar mass at the opposite 

 solstice. For, although (in consequence of the opposite state of the moon's declina- 

 tion at a given phase of the lunation) the lunar diurnal tide is different in sign, yet 

 the solar diurnal tide is also different in sign ; and thus the two diurnal tides are 

 mingled in the same degree at both solstices. The same applies if the observations 

 are at any opposite seasons of the year. 



It is possible to separate the two effects by comparing diurnal tides near a solstice 

 with diurnal tides near an equinox ; as, in the latter, the solar diurnal tide vanishes. 



Generally, it is possible to separate them by comparing two masses of diurnal tides 

 observed at intervals of three months ; as for the high (or low) waters corresponding 

 to a given right ascension and declination of the moon in the two masses, the sun 

 will have widely different positions in hour-angle, and therefore its effects at those 

 two instants will be widely different. 



The proportion of the effects of the sun and the moon cannot be ascertained from 

 a single series of observations, extending through a period so short that the sun's 

 position may be considered invariable. This will be shown by showing that the two 

 effects, of the sun and of the moon, in producing diurnal tide at high water, follow 

 sensibly the same law, and when added together give a compound effect following 

 the same law. Thus : tlie time of high water bears a nearly invariable relation to the 



