THEORY OF TIDES. 
should be confirmed in all its extent by observations made in 
a greater variety of situations than those which have hitherto 
been recorded. The advance of the spring tides at the sol- 
stices, which Laplace has adduced as an illustration of the de- 
pendence of the tides on the state of the luminaries a day and 
a, half before, is certainly favourable to his opinion. But this 
circumstance is not sufficiently established by continued obser- 
vation, to counterbalance the incompatibility of so slow a pro- 
gress of the tides from the open ocean with the well known 
times of high water at the different ports. Thete-is also some 
difficulty in explaining the occurrence of high water at St. He- 
lena hours, and at Bellisje l\ after the moon’s transit, in 
these situations, which seem to be as little remote as possible 
from the source of the tides of the Atlantic. The direct tides 
on the eastern coasts of a sea like the Atlantic, ought to happen 
about an hour before, and the inverted almost five hours after 
the transit : an acceleration of l\ hours seems to be greater 
than would be expected according to any probable estimate of 
the magnitude of the resistance. The simplest solution of 
this difficulty seems to be to suppose the deepest parts of the 
Atlantic much narrower than the whole of that ocean, so as to 
cause the simple inverted tide to happen considerably before 
the fifth lunar hour ; unless we choose to consider the principal 
part of the phenomena of our tides as dependant on the affec- 
tions of distant seas, so as to occur at times very different from 
those of the primitive variations of the Atlantic. The tides, 
on the western coasts agree very well with either supposition. 
The slight difference of the ascent and descent of the tide, ror 
marked by Mr. Laplace ip the observations at Brest, may be 
explained by a comparison with the form of a common wave, 
which, where the water is shallow, is always steepest before. 
This circumstance arises from the greater velocity with which 
the upper parts of tfie wave advance, where the difference of 
the depths becomes considerable, (Phil, T^ans. 1 80S) j audit 
fe, perhaps, somewhat increased, by the resistance of tfie bottom. 
Where the fide travels far in shallow channels, its irregularity 
pf inclination inctj eases more and for instance, in the 
Severn> 
