es north towards the
sea. They observe, in fact, that the water has two periodic motions--one
up and down, the other to and fro--a vertical and a horizontal motion.
They may further observe, if they take the trouble, that a complete
swing of the water, up and down, or to and fro, takes place about every
twelve and a half hours; moreover, that soon after high and low water
there is no current--the water is stationary, whereas about half-way
between high and low it is rushing with maximum speed either up or down
the river.

To both these motions of the water the name _tide_ is given, and both
are extremely important. Sailors usually pay most attention to the
horizontal motion, and on charts you find the tide-races marked; and the
places where there is but a small horizontal rush of the water are
labelled "very little tide here." Landsmen, or, at any rate, such of the
more philosophic sort as pay any attention to the matter at all, think
most of the vertical motion of the water--its amount of rise and fall.

Dwellers in some low-lying districts in London are compelled to pay
attention to the extra high tides of the Thames, because it is, or was,
very liable to overflow its banks and inundate their basements.

Sailors, however, on nearing a port are also greatly affected by the
time and amount of high water there, especially when they are in a big
ship; and we know well enough how frequently Atlantic liners, after
having accomplished their voyage with good speed, have to hang around
for hours waiting till there is enough water to lift them over the
Bar--that standing obstruction, one feels inclined to say disgrace, to
the Liverpool harbour.

[Illustration: FIG. 108.--The Mersey]

To us in Liverpool the tides are of supreme importance--upon them the
very existence of the city depends--for without them Liverpool would not
be a port. It may be familiar to many of you how this is, and yet it is
a matter that cannot be passed over in silence. I will therefore call
your attention to the Ordnance Survey of the estuaries of the Mersey and
the Dee. You see first that there is a great tendency for sand-banks to
accumulate all about this coast, from North Wales right away round to
Southport. You see next that the port of Chester has been practically
silted up by the deposits of sand in the wide-mouthed Dee, while the
port of Liverpool remains open owing to the scouring action of the tide
in its peculiarly shaped channel. Without