SEA- FISHERIES LABORATORY. 139 



to more than its. normal height : thus a stiff south-west breeze 

 south of Holyhead may cause the water on the Dock Sill at 

 Barrow to be a foot higher than the predicted tide, and a 

 north-east wind in the same southerly area cuts the tide in 

 Morecambe Bay even when, in the Bay itself, light westerly 

 breezes may be blowing. The amount of fresh water which 

 enters the Bay is also large : thus it is estimated* that the 

 rivers there carry down fresh water from about 1,080 square 

 miles of the wettest part of the British Islands. The quantity 

 of water that enters the Bay after heavy rains must alter the 

 temperature of the adjacent sea, especially after a time of 

 melting snow, and the volume of water setting out from the 

 Bay must also vary from this cause. These factors may 

 become more prominent ones in a " weak " year. 



These minor conditions are, to a great extent, incalculable 

 ones, and if a long series of years were considered, and com- 

 bined in some way, the irregularities that disturb the tidal 

 rhythm ought to " cancel out." With this idea in mind all 

 the data available have been treated in the following way : — 

 One tidal graph must be made from all the years. Now 

 the times of high water vary from year to year, but we can 

 take the graph of high waters for 1907 and superpose the 

 similar graphs for the other years upon this by shifting the 

 latter backwards or forwards until the maxima and minima 

 nearly coincide. Unfortunately, the method is complicated by 

 the fact that springs and neaps do not occur with the same 

 intervals in different years, and it is therefore necessary to 

 compare the dates of the highest springs and the lowest neaps 

 throughout, say, January to March for every year in order to 

 find the actual difference in days between the maxima and 

 minima of 1907 and those of other years. If the average of 

 these differences is obtained then one can make the closest 

 fit for the high- water curve of each year with that of 1907. 



* Report of British Rainfall Organisation for 1910. 



