302 
THE REV. W. WHEWELL ON TIDE OBSERVATIONS 
sponding to two such passages, the inequality increases from 0 to a maximum, and 
decreases to 0 again ; after which it again increases. 
The curves which represent the heights do, in fact, exhibit such alternate increase 
and diminution of the diurnal inequality : and the inquiry naturally occurs, After how 
long a time does the moon’s position show its effect in the diurnal inequality? In 
the case of Liverpool it appears, as I have pointed out *, that the diurnal inequality 
expresses the effect of the forces (upon the equilibrium-spheroid) as they existed six 
days previously. It is important to know whether this interval is the same in other 
places. 
25. It is very far from being the same, and its changes are very curious. In June 1835 
the moon had her greatest south declination on the 12th ; her declination vanished 
on the 19th, early in the morning; and her greatest north declination was on the 
26th. On the American coast, the diurnal inequality, as shown by the zigzag fofni 
of the curves, followed these changes, not at an interval of days, but almost simulta- 
neously. The curve is strongly indented from the 10th to the 15th : the indentations 
at most of the places die away on or about the 18th ; they then reappear, slipping 
over one tide, so as to throw the greatest tide from an odd to an even tide, or the 
reverse ; and increase to their greatest magnitude again about the 26th. On that 
side of the Atlantic, therefore, the difference of the lunar forces on the two successive 
half-days appears to be felt almost instantaneously. But when we come to the Euro- 
pean shore the result is very different. On the coasts of Spain and Portugal, and on 
the coast of France as far eastward as Cherbourg, the diurnal inequality is very steady 
and well marked, but it only appears to begin about the 9th or 10th, increases till 
the 16th or 17 th, then decreases, and vanishes on the 21st or 22nd, after which it 
again increases. Thus the moon’s crossing the equator on the 19th is not felt in its 
effects till two or three days afterwards. In like manner, on the coast of Cornwall, 
and on the west coast of Ireland, the inequality is well marked till the 21st or 22nd, 
after which it vanishes, and reappears irregularly only. As we advance further in the 
direction of the progress of the tide, we find the epoch of the diurnal inequality to be 
later and later, although the inequality, and therefore its epochs, are less clearly 
marked. Thus at Cowes, Portsmouth, and Hayling Island, the inequality begins on 
the 13th and vanishes again on the 23rd; on the east coast of Scotland, and of the 
North of England, in like manner, it appears on the 12th or 13th; but it seems to 
pass over a tide, which is equivalent to its vanishing, as early as the 21st. In the 
German Ocean, however, its course is not very intelligible ; for though it appears 
very marked in the Danish observations, from the 12th to the 22nd, it misses one tide 
on the 18th. As the Danish tides will be seen by the map to arrive by two different 
paths, one of which is half a day longer than the other, it is easy to explain this 
change in the regular alternation of the tides, by supposing that the tide which 
comes from Scotland was predominant at one period of the lunation, and that which 
, * Philosophical Transactions, 1836, Part I., page 97. 
