MADE ON THE COASTS OF EUROPE AND AMERICA IN JUNE 1835. 
305 
shallow, an addition of two feet may make the water advance many hundred yards 
further ; and thus, on this account, the time of high water would be later. The diur- 
nal inequality of the heights, therefore, will depend upon local circumstances, not 
only for its quantity, but for its sign. 
It appears by the observations that the diurnal inequality of the times is the most 
clearly marked in situations where the mixture of two tides ends ; as at the north- 
east point of Ireland, where the tide following the a.m. transit of the moon is later 
than the mean ; at the south-east point of Ireland, where the tide following the a.m. 
transit is the earlier ; at Ostend ; at Havre ; on the coast of Denmark, where this 
diurnal inequality amounts to half an hour. The diurnal inequality is also very large 
in places where the tide has to run far inland, as in the Sound of Christiania in Nor- 
way, and in the Zuyder Zee in Holland. At Amsterdam the difference resulting 
from this inequality appears to be an hour ; in the neighbourhood of Christiania it is 
larger still, but with great anomalies. 
Sect. VI. On the Semimenstrual Inequality. 
29. The amount of the semimenstrual inequality of the time of high water is very 
different at different places, so far as the evidence of the observations now before us 
shows ; and though these are of too rude a kind to give the amount of the difference, 
they are sufficient, I think, to prove its existence ; especially when coupled with the 
consideration of a reason for the difference, namely, that the spring tides being 
higher than the neaps, the tides of the two kinds may travel with velocities which at 
different places have different relations. Thus I conceive that I have here a confirma- 
tion of the opinion which I deduced from the observations of June 1834, that there 
is a local semimenstrual inequality in addition to the general one*. But I do not 
conceive that this series offers any very decisive proof of my former conjecture, that 
the semimenstrual inequality is less at promontories than in bays, or that it becomes 
less and less as the tide-wave advances. The changes of this inequality are not ob- 
viously explicable. On the coast of North America the amount of the difference of 
the greatest and least lunitidal intervals is small, being generally less than 80 ra , and 
at Newport as low as 56 m . On the coast of Portugal at several places this difference 
is extraordinarily small, so as almost to throw doubt on the accuracy of the observa- 
tions : at Pera in Algarve it is only 42 m , and at Lagos Bay only 24 m , while at 
Peniche it is 130 m . On the greater part of the French coast it ranges with great 
steadiness from 80 m to 100 m , except at the little harbour of Ab re vrak, where it is 125 ni . 
At Torr Head (in the north-east of Ireland) we have this difference 146 m , and at 
Rachlin Island (North of Ireland) it is four hours, even after the graphic correction ; 
but these are cases of extreme irregularity. On many parts of the south coast of 
England it is small (about 70 m to 74 m J, as at Exmouth, Weymouth, St. Alban’s Head, 
St. Lawrence, Swanage Bay, Brighton, and Hastings. 
* Philosophical Transactions, 1835, p. 85. 
2 R 
MDCCCXXXVI. 
