230 
THE REV. W. WHEWELL ON THE PROGRESS OF THE 
vanished on the 6th and on the 18th. Thus the epoch is the same, or nearly the 
same, at all these places, namely, about four days, which is the value I had already 
assigned to it from several years’ observations at Plymouth. It appears to be half a 
day, or perhaps a day greater than this on the east coast of Britain, but on that coast 
the tide has been from half a day to a day longer in arriving ; so that we have here 
nothing to favour the opinion that the diurnal inequality is transferred at a different 
rate from the other inequalities of the tides, and the suggestions contained in my last 
memoir respecting the laws and causes of the supposed peculiar movement of the 
diurnal inequality must be rejected. They were founded principally on observations 
made at Leith, in which the diurnal inequality was very imperfectly exhibited ; the 
rejection of them is founded on observations made at sixty-five places, taken in order 
along the coasts of England, Ireland, and Scotland ; for I have examined the diurnal 
inequality at many places on those coasts, besides those for which I have drawn the 
curves of the first series, and I find a general agreement in the features of contiguous 
places. 
The slow propagation of the diurnal inequality from day to day being thus rejected, 
we have next to consider the motion of the diurnal wave for each day, by means of 
our curves. It will be observed that in each curve the alternate strong ordinate lines 
belong to high waters, and the intermediate lines to low waters. The maxima of the 
swells of the curves, above and below the axis, are the summits of elevation and de- 
pression of the diurnal wave, and by the position of these summits with regard to 
high and low water, we see whether the diurnal wave arrives before or after the semi- 
diurnal wave, and by how much. And as we know at each place how long the semi- 
diurnal wave arrives after the moon’s transit, we thus can refer the diurnal wave to 
the moon’s transit. 
In doing this we must make a distinction between the superior and inferior transit, 
which does not affect the semidiurnal waves. The diurnal wave belonging to the 
superior transit will (by theory) increase the tide when the moon’s declination is 
north, and diminish the tide when the declination is south. Hence if we consider 
our diurnal wave as belonging to the moon’s superior transit, since from June 6th to 
June 18th, 1835, the moon’s declination is south, we must take the lower summits 
of the curve from June 10th to June 22nd ; and the mode of proceeding is obvious. 
At Ferrol, for example, it will be seen that the lower summits of the diurnal wave 
occur in general about two hours before the high water ; and this is the high water 
which follows a superior transit; for on June 16th, for instance, it is the morning 
tide which occurs at 6 h 42 m , the moon’s transit occurring June 16th 4 h 53 m a.m. Now 
the tide at Ferrol, taking the average interval (the “corrected establishment” of my 
former Researches), follows the moon’s transit at an interval of 2 hours and thirty 
minutes. Therefore the diurnal wave at Ferrol follows the moon’s superior transit 
at an interval of thirty minutes. 
The following is the result of the investigation in this series of places. 
