DIURNAL INEQUALITY OF THE HEIGHT OF THE TIDE. 
83 
of disappearing ; and the progress of this alternation affects the tides as much as the 
independent alternation of springs and neaps. 
It is easy to conceive the diurnal inequality carried a little further than it is at 
Singapore ; so that at a certain stage of it the alternate tides would vanish. This is 
equivalent to supposing the highest low water and the lowest high water to have the 
same height. 
There are statements of navigators respecting various places at which there is 
“ only one tide in twenty-four hours.” From what has been said it appears that this 
may happen during a part of each semilunation by the effect of the inequality now 
under consideration, but that it cannot in this way be constantly the case. 
I am fortunately enabled to throw some light on this subject by the kindness of 
Captain Fitz Roy. King George’s Sound on the south coast of New Holland is one 
of the places to which these Single-day Tides have been ascribed*. In March 1836 
Captain Fitz Roy, aware of the interest of this position in respect to tide phenomena, 
caused observations to be made every half hour for some days, and for a portion of 
the time, every quarter of an hour. The result w'as that on March the 7th and 8th 
there were two very unequal tides, and that on the 9th and 10th there was only one 
tide ; but a recession and return in the high water, which had been barely perceptible 
on the 11th, became more and more marked on the 12th, 13th, and 14th, so as again 
to give two tides each day. Thus at this place it appears to be only at one particular 
period of the semilunation that we have a single-day tide, agreeably to our general 
view. I insert the curve of the motion of the surface at King George’s Sound in 
Plate IV. 
The single-day tides of Tonquin~f~ were referred by Newton to the interference of 
two tides, which arrive by different channels. The great diurnal inequality of Sin- 
gapore, which is in the same seas, appears to be clearly due to the effect of the moon’s 
declination ; and the establishment of this point, and the circumstances ascertained 
to occur in the reputed single-day tide of King George’s Sound, throw some doubt 
on the explanation just referred to, which cannot be removed till the tides of those 
seas have been more fully observed. 
Sect. V. On the Mean Height of the Sea. 
The question of the fluctuations of the mean height of the sea is not especially con- 
nected with the diurnal inequality. But as the curves which I had to draw in the 
course of this investigation give me the means of exhibiting very clearly these fluc- 
tuations, I will here say a word on the subject. 
In Plate II. a line is drawn representing the mean height ; that is, the height 
midway between low water and high water each day. It is obtained by taking the 
mean of the two curves of high and low water by which the diurnal inequality is cut 
off. The same is done for Singapore in Plate III. 
* Flinders, vol. i. p. 71. King, vol. ii. p. 380. f See Philosophical Transactions, vol. xiv. p. 162. 
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