44 



MR. AIRY ON THE LAWS OF THE TIDES 



In the Tides and Waves, Art. 463 and 465, I have shown that the age of tide in- 

 ferred from the times of higli water in a river (where spring-tide high waters pass 

 more rapidly than neap-tide high waters) is too small, and that the age of tide in- 

 ferred from the times of low water in a river (where spring-tide low waters pass more 

 slowly than neap-tide low waters) is too great. These propositions, at least the 

 second, are well illustrated at Limerick and New Ross. For the other stations I 

 feel myself in some difficulty. With the exception only of Old Head, Dunmore East, 

 and Passage West, all the ages of tide above are too small, for low water as well as 

 for high water. This requires us to assume that all the phases of the tide-wave (low 

 water as well as high water) are transmitted over the sea more rapidly in the spring- 

 tides than in the neap-tides. I conjecture that some theory of friction may possibly 

 explain this. It cannot be explained by supposing the second power of the small 

 movements sensible ; for on that assumption the age of tide given by low water 

 would be increased. 



It is worthy of remark that at Ballycastle the effects depending on the position of 

 the sun and moon appear to precede their cause. 



Section IX. — Formation of the time of diurnal high water ; progress of the diurnal 

 tide-wave i^ound the island; comparison of its progress and range with those of the 

 semidiurnal tide. 



In page 20 I have given a table of the maximum values of diurnal tide, at high 



water and at low water. The diurnal tide being supposed to follow the law of sines, 



its maximum coefficient will be found by taking the square root of the sum of the 



squares of those two values, and the time after semidiurnal high water at which 



diurnal high water occurs will be found by taking the angle whose tangent 



value at low water . . , , . . i . ^ „^^r. f 



^ value at high water ' ^ converting that angle into time at the rate or 360 for a 



lunar day. The maximum diurnal tide for semidiurnal high water and that for semi- 

 diurnal low water may be conceived to hold for any day near to the day of absolute 

 maximum. Thus the following Table is formed. 



