ME. WILLIAM PAEKES ON THE TIDES OE BOMBAY AND KUEEACHEE. 687 
series of semidiurnal tide-waves present in their variations in height and time a nearly 
unbroken gradation. 
When, however, either luminary is out of the plane of the equator, the tide formed on 
the nearer side is different from that formed on the further side of the earth ; and the 
difference depends, not upon its origin being on the further or nearer side of the earth, 
but upon its being situated in the northern or southern hemisphere. Such of the 
variations in the tidal movements, therefore, as are due to the fact of their having taken 
their ultimate origin from the northern or southern hemisphere, recur every day instead 
of every half day ; and hence we have a result which, if it could be separated from the 
semidiurnal features of the tide, would appear as a diurnal tide. 
I have now to show how this supposition of an independent diurnal tide agrees with 
the observed tidal movements. 
The diurnal tide being a combination of solar and lunar effects, is subject to the 
recurrence of neaps and springs like the semidiurnal tide, but beyond this it is subject 
to a recurrence of changes as the luminaries leave and reapproach the plane of the 
equator. The semidiurnal luni-solar tide is a combination of two tides of different 
periods, but each in itself of dimensions varying only slightly. The diurnal luni-solar 
tide, on the contrary, is a combination of two tides not only of different periods, but each 
in itself of ever-varying magnitude. 
Again, it is well known that the time of semidiurnal tide varies to some extent during 
the half lunation, according as the solar portion of the tide precedes or follows the lunar 
portion ; but as the lunar effect preponderates in the nearly constant ratio of about 3 to 
1, a tide occurs once in every lunar half day. In the diurnal tide, however, the lunar 
influence does not always predominate. Twice in the course of a tropical month, when 
the moon is on the equator, her diurnal effect vanishes, and the period of diurnal tide 
then responds to the sun’s influence alone. After the moon has crossed the equator, 
however, her influence rapidly increases, and the diurnal tide then again follows the 
lunar period. But the lunar effect, in thus recovering its preponderance, does so under 
changed conditions. The times of lunar diurnal high water and low water are now 
reversed. Thus there is an abrupt transition of twelve hours from the time of the 
vanishing tide to that of the reappearing one. The abruptness, however, is more or 
less counteracted by the solar diurnal tide, which maintains its regular periods. From 
these considerations it will be evident that, whether in respect of height or time, the 
diurnal features of the tide are of a far more complicated character than the semidiurnal 
features. 
In considering the effect of the combination of the semidiurnal and diurnal tides, it 
will be obvious that if one diurnal high water takes place at the same time as a semi- 
diurnal high water, the next diurnal low water will take place at the same time (or 
nearly so) as the next semidiurnal high water. Thus there will be a difference between 
the two successive semidiurnal high waters equal to the whole range of diurnal tide. 
Since, however, in the first semidiurnal tide the water ceases to rise at the same time as 
