30 



MR. AIRY ON THE LAWS OF THE TIDES 



For each of these groups the mean of the heights at high water was taken, and the 

 mean of the heights at low water was taken. Then the mean of the determinations 

 of each of these classes in the 1st, 3rd, 5th, and 7th groups was taken, and thus was 

 obtained a mean height at high water and a mean height at low water in small tides. 

 The mean of these two means gave the Apparent Mean Height in small tides (so called 

 in order to distinguish it from the Mean Height which will be found in Section XL). 

 By a similar treatment of the 2nd, 4th, and 6th groups, the mean height at high 

 waterj mean height at low water, and Apparent Mean Height, in large tides, were 

 found. The results are as follows : — 



The column which first demands our attention is the " Mean of Apparent Mean 

 Heights." The Apparent Mean Height increases in ascending the Shannon from 

 Kilbaha to Limerick, and in ascending the Barrow from Dunmore East to New Ross. 

 There is no difficulty in perceiving that such an effect must result from the mixture 

 of a river-current with a tide, when the motion of the water is impeded by friction. 

 But the Apparent Mean Height increases also from Kilbaha and Castle Townsend in 

 going northward along both the eastern and the western coasts. And this occurs 

 both at large tides and at small tides, with comparatively very little difference of 

 amount. This conclusion will be fully supported by the results deduced by a more 

 elaborate process from the whole mass of observations in Section XI. The details 

 of levelling in the first section will scarcely permit us to ascribe any large part of 

 this seeming difference of Apparent Mean Height to error in the instrumental process. 



