294 
THE REV. W. WHEWELL ON TIDE OBSERVATIONS 
are real and simultaneous observations at a sufficient number of places along- the 
coasts, gives them an immense superiority over the statements which I was formerly 
compelled to use, and which were for the most part only estimated results, founded 
upon imperfect observations or none, and often deduced by erroneous methods of 
estimation. 
It is not surprising, therefore, that the differences between the form of the lines 
now obtained and my former maps should be considerable. At the same time I may 
observe, that all my views of the general course of the tide-wave have been confirmed 
by the present examination. 
11. With regard to the general character of the corrections which I have had to 
introduce into my maps, I may state this as one circumstance : the cotidal lines make 
very acute angles with the shore, and run for great distances nearly parallel to it. I 
had already, to a certain extent, pointed out that the cotidal lines must have a shape 
of this kind. “ They are convex,” it was observed *, “ in the direction of their mo- 
tion, the ends near the shore being held back by the smaller velocity in shallower 
water, and other resistances.” But it is necessary to exaggerate very much this 
feature in their shape, in order to make them conform to our observations, so that 
the lines near the shore are made near and almost parallel to each other. In this 
way the velocity of the tide-wave, which is, of course, to be estimated in a direction 
nearly perpendicular to the cotidal lines, is very much less near the shore than it is 
in the open ocean : perhaps we may even consider the velocity of the tide-wave in 
littoral regions as a quantity of a different order, and governed by different laws, 
from its velocity in the open ocean : but of this we may speak more distinctly here- 
after. 
One consequence of this form of the cotidal lines is, that though on a large extent 
of coast the direction and velocity of the progress of the tide-wave are marked clearly 
enough, in smaller portions the rate and even the direction of this progress may 
rapidly and repeatedly change. The cotidal line leaving the shore at so small an 
angle, may easily catch it again where it projects a little, and thus we have points of 
divergence and of convergence of the cotidal lines -j\ 
For example, on the coast of America (see Table I.) the progress of the tide from 
Cape Hatteras is both southward to Cape Fear, Charlestown, Savannah, and St. Au- 
gustine, and northward to Delaware and New York ; Cape Hatteras being a point of 
divergence. But at Newport, still further to the north-east, we find the tide again 
an hour earlier than New York, and even earlier than at Delaware Breakwater ; so 
that between Cape Hatteras and Newport there must be a point of convergence. To 
the east of this, again, there is a point of divergence, and the hour of the tide becomes 
rapidly later as it travels into the bays of Massachusetts, Boston, and Fundy. 
In the same manner, on the coast of Spain (see Table II.) the 2 h line touches the 
shore near Cadiz ; it also touches at Cascaes near Lisbon, the tide-hour at interme- 
t Ibid., p. 153. 
* Philosophical Transactions, 1833, p. 231. 
