AND ON THE DIURNAL INEQUALITY. 
29 
Swan River, on the west, it principally affects the times. And the diurnal inequality 
which alters the low water four feet at Port Essington, and six feet at Sincapore, 
affects the high water to a still greater extent in the Gulf of Cambay, and disturbs 
the times at the entrance of the Persian Gulf. 
Where the diurnal inequality affects the heights so much, it probably affects mate- 
rially the time of tide in all places, as we know it does in some. And therefore 
over the whole of the Indian seas, the terms “establishment” and “ tide-hour” must, 
without further explanation, be very ambiguous. And as we cannot know what facts 
are designated by such terms in books of navigation, all attempts to found upon 
such statements a coherent view of the tides of those seas, must be very precarious*. 
.52. Upon the occasion of the great series of tide observations made in 1835, 1 was 
led to trace the progress of the diurnal inequality wave along the coast of Europe. 
This I attempted in the Eighth Series of these Tide Researches -f-. The difficulty of 
separating the diurnal from the semidiurnal wave was there in some degree overcome. 
But even in that case, the problem was very imperfectly solved ; for the coasts of the 
Pacific, and even for the coasts of Australia, our materials are too scanty and discon- 
nected to give us any hope of success at present, if we were to attempt the same 
problem. With due materials, the diurnal inequality appears plainly to be separable 
from the semidiurnal tide ; for it sometimes affects high water most, sometimes low 
water; sometimes the heights, sometimes the times; and is large sometimes when 
the semidiurnal tide is small, and sometimes exceeds the semimensual inequality. 
53. It was remarked also, on the occasion of the observations of 1835:}:, that the 
diurnal inequality on the coast of North America followed the changes of the moon’s 
declination almost instantaneously ; while on the coasts of Portugal, Spain and France, 
the changes of lunar declination were represented in the diurnal inequality two or 
three days later ; and at the Cape of Good Hope, about the same time. I have 
already noticed that this feature throws great difficulty in the conception of that mo- 
tion of the waves by which the tides are produced. It suggests the necessity of some 
new mode of conceiving that motion ; a subject which I shall not here pursue. 
* Philosophical Transactions, 1833, p. 200. t Ibid. 1837, Part II. J Ibid. 1836, Part II. p. 302. 
Trinity Lodge, Cambridge, 
Nov. 8, 1847. 
