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VII. Researches on the Tides —Seventh Series. On the Diurnal Inequality of the 
Height of the Tide, especially at Plymouth and at Singapore ; and on the Mean 
Level of the Sea. By the Rev. W. Whewell, M.A. F.R.S., Fellow of Trinity 
College, Cambridge. 
Received March 7, — Read March 9, 1837. 
THE Inequality of the Tides which is the subject of the present paper, though 
theoretically very curious, and practically very important, has hitherto been hardly 
noticed, and its laws have never been generally stated. By means of the materials 
which I have had in my hands, I have not only been able to obtain a rule agreeing 
with the observations to an extraordinary degree of precision, but I have found and 
analysed a case in which this inequality assumes a very remarkable form, so as mate- 
rially to disguise the general circumstances of the tides, and to explain other cases 
in which the usual features are entirely obliterated. 
The inequality of which I speak is the Diurnal Inequality, by which the tide of the 
morning and evening of the same day differ. The difference is often very consider- 
able, especially in the height of the water ; and naval officers have often found the 
preservation or destruction of a ship to be caused by this difference, without being 
aware that it was subject to steady rules, and was capable of being predicted. The 
small number of places for which I have been able to procure the proper observations 
will not permit me at present to state the circumstances of the inequality as they 
occur all over the surface of the ocean ; but I am, by fortunate circumstances, able 
to trace its laws in some very remarkable instances, situated in very widely separate 
regions of the globe. 
Sect. I. Diurnal Inequality at Plymouth. 
I will first treat of the diurnal inequality as it appears at Plymouth, at which port 
good tide observations are regularly made under the direction of Mr. Alexander 
Lumsdale and Mr. William Walker, the Master Attendant and Assistant Master 
Attendant of the Dock-yard. 
It has long been known that both at Plymouth and at other places there is com- 
monly a difference in the morning and evening tide of the same day. It is stated by 
Colepress in 1668*, that at that port “ the diurnal tides from about the latter end of 
March till the latter end of September are about a foot higher in the evening than in 
the morning ; that is, every tide which happens after twelve in the day before twelve at 
* Philosophical Transactions, vol. iii. p. 633. 
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