DIURNAL INEQUALITY OF THE HEIGHT OF THE TIDE. 
85 
Postscript. 
I will take the liberty of mentioning the only way in which it appears to me 
mechanically possible to conceive the slow propagation of the diurnal inequality 
which I have described in Sect. III. 
If we suppose equal semidiurnal tides to be propagated along the length of a wide 
canal ; and if we suppose, in addition to these, a transverse oscillation of the water to 
take place in the direction of the width of the canal, the time of this oscillation 
(from maximum to maximum) being a whole tide day; we shall have successive tides 
alternately greater and less by a diurnal inequality. And we may easily suppose this 
transverse oscillation to be propagated gradually and slowly along the canal, by the 
contact of the particles of the water. In this manner we may represent phenomena 
following laws like those above described. 
But it may be further observed, that we may conceive the semidiurnal tide, as well 
as the diurnal inequality, to be propagated along the canal by means of transverse 
oscillations, the time of this oscillation being half a lunar day ; and the rate of pro- 
pagation of this undulation may easily be supposed to be different from that of the 
diurnal oscillation. In this way we may conceive the possibility of the different in- 
equalities of the tides being propagated from place to place at different rates, and 
thus having different epochs, as from the recent researches on the subject contained 
in the Philosophical Transactions they appear to have. 
Moreover, it is by no means necessary, in order to make this explanation applicable, 
that the transverse undulations should be perpendicular to the direction in which the 
tide is propagated : they may be oblique to it at any angle, and the result will still 
be the same. 
It appears possible, also, that such a supposition may be modified, so as to explain 
other phenomena of the tides ; for instance, the smallness of the tides in the central 
parts of wide seas. 
But the application of such a supposition to the actual phenomena of the ocean, 
and the determination of those tracts of sea which must, on this view of the case, be 
looked upon as tide-canals, would be a matter of no small difficulty, even if our ma- 
terials were sufficient for the purpose , and would probably be impossible without 
more knowledge of the tides on the shores of the great oceans than has yet been 
published. 
Trinity College , Cambridge, 
May 5, 1837- 
