ends on whether the place has the general character of a
terminus, or whether it lies _en route_ to some great basin.

You must understand, then, that all tide takes its rise in the free and
open ocean under the action of the moon. No ordinary-sized sea like the
North Sea, or even the Mediterranean, is big enough for more than a just
appreciable tide to be generated in it. The Pacific, the Atlantic, and
the Southern Oceans are the great tidal reservoirs, and in them the
tides of the earth are generated as low flat humps of gigantic area,
though only a few feet high, oscillating up and down in the period of
approximately twelve hours. The tides we, and other coast-possessing
nations, experience are the overflow or back-wash of these oceanic
humps, and I will now show you in what manner the great Atlantic
tide-wave reaches the British Isles twice a day.

[Illustration: FIG. 109.--Co-tidal lines.]

Fig. 109 shows the contour lines of the great wave as it rolls in east
from the Atlantic, getting split by the Land's End and by Ireland into
three portions; one of which rushes up the English Channel and through
the Straits of Dover. Another rolls up the Irish Sea, with a minor
offshoot up the Bristol Channel, and, curling round Anglesey, flows
along the North Wales coast and fills Liverpool Bay and the Mersey. The
third branch streams round the north coast of Ireland, past the Mull of
Cantyre and Rathlin Island; part fills up the Firth of Clyde, while the
rest flows south, and, swirling round the west side of the Isle of Man,
helps the southern current to fill the Bay of Liverpool. The rest of the
great wave impinges on the coast of Scotland, and, curling round it,
fills up the North Sea right away to the Norway coast, and then flows
down below Denmark, joining the southern and earlier arriving stream.
The diagram I show you is a rough chart of cotidal lines, which I made
out of the information contained in _Whitaker's Almanac_.

A place may thus be fed with tide by two distinct channels, and many
curious phenomena occur in certain places from this cause. Thus it may
happen that one channel is six hours longer than the other, in which
case a flow will arrive by one at the same time as an ebb arrives by the
other; and the result will be that the place will have hardly any tide
at all, one tide interfering with and neutralizing the other. This is
more markedly observed at other parts of the world than in the British
Isles. Wh