86 THE REV. W. WHEWELL'S RESULTS OF TIDE OBSERVATIONS. 



By the introduction of a local semimenstrual inequality, in addition to the general 

 semimenstrual inequality, we should be able to reconcile the discrepancies of the 

 curve which represents this inequality for different places, as London and Plymouth ; 

 discrepancies which have hitherto been a source of perplexity to those who have stu- 

 died the subject. 



The existence of these discrepancies, and their general prevalence, which is shown 

 by our observations, make it clear that we cannot correctly use the tide table of one 

 place to determine the tides of another, by adding or subtracting a constant interval, 

 as is often done. For such a purpose the difference of the local semimenstrual in- 

 equalities of the two places requires also to be applied. 



I have not attempted to determine the amount or form of the local semimenstrual 

 inequalities of different places, not thinking our present materials sufficient ; but a 

 comparison of the curve of the semimenstrual inequality of different places is the 

 way in which it must be obtained, and on this subject I have some remarks to make. 



6. By what causes is the semimenstrual inequality at one place made to be dif- 

 ferent from that at another? There are some circumstances which we can readily 

 imagine may produce such an effect, though we should probably not succeed in 

 guessing what the effect would be ; as, for instance, the form of the coast, the di- 

 stance which the tide wave has travelled over, and the meeting of tides proceeding 

 different ways. I think I can discover in the observations of last June indications of 

 the effect of all these circumstances. 



In the first place it appears that the curves (by which I mean here, and in what 

 follows, the curves of the semimenstrual inequality) are Jiatter when the observations 

 are made at promontories than they are for the general line of coast. I speak here of 

 the promontories of the first order, which divide the south coast of England into large 

 or primary bays, as the Lizard, the Rame Head, Prawle Point, Portland Bill, St. Al- 

 ban's Head, St. Catherine's Point, Beachy Head. At such places the amount of the 

 semimenstrual inequality appears from the observations to be less than it is in the 

 intervening bays. 



7. In the next place it seems to follow from the observations that the curves are 

 flatter and flatter as the tide wave proceeds further and further. Thus the curve is 

 flatter in Brighton Bay than it is in Mount's Bay on the coast of Cornwall, the tide 

 having travelled further from west to east. 



I do not consider this point as quite firmly established, because, though the curves 

 do exhibit such a modification in going eastward, when we get as far east as the Isle 

 of Wight we seem to perceive the influence of another cause which has been men- 

 tioned, the meeting of the two tides, which may produce this apparent modification. 



8. This subject, the meeting of the tides, appears to be often misunderstood. For 

 instance, in a paper published in the Philosophical Transactions for 1819, it is taken 

 for granted, that when the two tides meet which come up the British Channel and 

 down the German Ocean there must be a visible and marked conflict of opposite cur- 



