- 
304 THE REV. W. WHEWELL ON TIDE OBSERVATIONS 
all through the month, as has been stated for Plymouth and other places on various 
occasions. We now see that this is merely an accidental result of the true rule. 
27. The different epoch of the diurnal inequality in different parts of the world is a 
very curious fact ; and the more so, since it is inconsistent with the mode hitherto 
adopted of explaining the circumstances of the tides by conceiving a tide-wave to 
travel to all shores in succession. In accordance with this view the tide on the 
shores of America had been considered as identical with the tide on the coasts of 
Spain and Portugal, which occurs about the same moment ; nor does it appear easy 
to imagine the form of the tide-waves so that this shall not be the case. Yet we find 
that the tides on these two sides of the Atlantic cannot be identical in all respects ; 
for on the 9th, 10th, and 11th of June, when the diurnal inequality was great in 
America, it was nothing in the West of Europe ; and on the 18th and 19th, when this 
inequality had vanished in America, it was great in Europe. It would seem as if the 
tidal phenomena on this side of the Atlantic corresponded to an epoch (of the equi- 
librium-theory) two or three days later than the same phenomena in America ; and 
we may perhaps add, that different kinds of phenomena do not appear to travel at 
the same rate. And thus the equilibrium-theory, though it may explain the general 
form of the inequalities, cannot give their epochs and amounts by any possible ad- 
justment of constants. 
I may add, that the notion of the progress of the tide- wave from south to north in 
the Atlantic is still further involved in difficulties by its appearing that at the Cape of 
Good Hope the diurnal inequality showed itself most clearly on the 17 th, 18th, and 
19th of June ; that is, as late as in Spain and Portugal. This appears by observations 
undertaken at my request by Sir John Herschel ; and though these observations, 
made under very inconvenient circumstances, are not very regular, there can, I think, 
be no doubt of the reality of the feature to which I have referred. 
28. The diurnal inequality appears also, but not so generally, in the curves which 
represent the times : nor is this difference always in the same direction. Thus on the 
coast of America, at some places the p.m. tides are later than the mean, and the a.m. 
earlier, for a great part of June 1835, while at other places the reverse is the case : 
and the same peculiarity occurs on other coasts. 
Though this circumstance appears at first sight anomalous, it is not difficult to ex- 
plain it, at least hypothetically. The alteration of the time of high water by means 
of the diurnal inequality results, not only directly from the change of position of the 
equilibrium-tide, which of course affects all places alike, but also indirectly, from the 
diurnal inequality of the height ; for tide-waves of different heights may both travel 
with different velocities, and have different spaces to describe : and thus the conse- 
quent change of time may either tend to make it sooner or later. If the evening tide 
be two feet higher than the morning tide, it may on that account travel faster along 
that part of the channel which they have in common ; but then, if the shore be very 
