22 
THE REV. W. WHEWELL ON THE TIDES OF THE PACIFIC, 
I add a very slight and imperfect sketch of the cotidal lines of the Pacific as they 
result from the materials just examined. (See the Plate.) 
T'he Diurnal Inequality. 
40. There is a feature in the tides, very important both in its practical elfect and 
in its bearing upon the theory, which has long been, in some degree, known to navi- 
gators, but of which they do not seem sufficiently to appreciate the generality, and of 
which they have commonly mistaken the law. I speak of the difference of the two 
tides on the same day, which I have termed the Diurnal Inequality. It was noticed 
at Plymouth and at Bristol as long ago as the time of Newton, and was, to a certain 
extent, explained by him. It was shown by Laplace to exist in the tides at Brest. 
It was found by Captains Cook, Flinders and King, to be very large on the coasts 
of Australia. Its amount at Sincapore is enormous. Admiral Lutke found it to 
obtain in all his observations in the North Pacific. It appears in most parts of the 
Atlantic ; and it is very considerable at the Falkland Isles, at Cape Horn, at New 
Zealand and at Kerguelen’s Island. Indeed the cases where it does not occur are 
the exceptions, and commonly belong to shores where two tides are combined, as in 
the east coast of England, where it is nearly obliterated. 
41. I have said that the laws of this inequality have commonly been mistaken. 
Thus navigators have spoken of the difference of day tides and night tides. Captains 
Cook, Flinders and King, say that on the coast of Australia the night tides are always 
greater than the day tides. I have shown* that the seeming truth of this assertion 
was occasioned by the time of year at which those navigators respectively made their 
observations. Lieut. Wilkes'I' says that on the coast of the Nisqually Indians in the 
Oregon Territory, the day tides during his*observations were two feet higher than 
the night tides. This also was only a temporary rule. No reference of the inequa- 
lity to day tides and night tides, or morning tides and evening tides, can express its 
law. It depends upon the moon’s declination, and changes to alternate tides when 
the moon’s declination changes from north to south, and vice versa. Its rule is ex- 
pressed in the following form^ 
For moon’s N. declination, j following moon’s South transit, 
I Subtract from the tide following moon’s N. transit. 
For moon’s S. declination. | f™” ‘he tide following moon’s S. transit, 
I Add to the tide following moon’s N. transit, 
the quantity added or subtracted being greater as the declination is greater ; and the 
declination being taken for one, two, or three days previous to the tide. According 
to this law, the inequality has been introduced into the Tide Tables for Liverpool, 
Bristol and Plymouth. 
It may be worth while to show that the rule of the diurnal tide which has just 
* Philosophical Transactions, 1833, p. 221. f United States Exploring Expedition, iv. 417. 
I Philosophical Transactions, 1837, p. 84. 
