AND ON THE DIURNAL INEQUALITY. 
5 
sequence of the tide-hours along- the shore may be rightly represented by cotidal 
lines drawn nearly parallel to the shore, according to the data which observation 
supplies. 
14. It is to be observed, however, that the oceanic tide necessarily includes two 
different areas, in which the times of high water differ by six lunar hours ; and 
therefore that which is assumed as the oceanic space, must be so situated that the 
tides on its opposite sides differ by about six lunar hours. The oceanic space thus 
occupied by the stationary undulation must, further, be so situated that the tides in 
its middle parts are small or disappear. 
15. But a stationary undulation, such as has been supposed to occupy the oceanic 
space, is not the only mode in which we may conceive large littoral tides combined 
with small central tides. Such a combination may be produced by cotidal lines re- 
volving round a fixed centre, as in the upper part of figure 3. 
Nor is such a state of things imaginary only. The course 
of the tides on the opposite coasts of England and the Ne- 
therlands is such as not to be intelligible in any other 
manner than by supposing such a rotation of the tide-wave, 
as may be seen in my Sixth Series of Tide Researches. 
And the smallness of the tide in the central parts, which this 
view of the subject implies, has been verified by the obser- 
vations of Captain Hewett and others. A case somewhat 
of the same kind occurs on the coast of Ireland. At Cour- 
town on the east coast, there is no lunar tide*, though there 
is a progressive tide-wave on the opposite coast of Wales. 
16. It may be thought, therefore, that we shall find it impossible to decide whether 
the tides of an ocean in which the central tides are small and the littoral tides pro- 
gressive, are to be represented by a revolving wave or by a stationary oceanic undu- 
lation with bordering cotidal lines. But this difficulty is not of much real consequence; 
for one of these hypotheses passes gradually into the other, as may be seen in fig. 3. 
And the result of both the one and the other is, that we cannot pronounce anything 
eertain about the time of high water in the oceanic space, till we have been enabled, 
by numerous observations, to draw the littoral cotidal lines with considerable accu- 
racy ; and when this is done, the nature of the oceanic movement will probably 
show itself upon the face of our chart. 
17 . On these grounds, I am now disposed to retract parts of what I have said with 
regard to the form of the cotidal lines of the Atlantic in my “ Essay.” I do not think 
it likely that the course of the tide can be rightly represented as a wave travelling 
from south to north between Africa and America. We may much better conceive the 
state of things by means of a stationary undulation, of which the middle space is 
between Brazil and Guinea, in which region the tides are very small, as at St. Helena 
* See Mr. Airy in the Philosophical Transactions, 1845, Part I. 
Fig. 5. 
