should be kept by each observer for reference, one for
dispatching a message arranged in alphabetical order and the
other far reading a message arranged as set out above. The
white flag should be used when standing against a dark
background, and the blue one when on the skyline or against a
light background.

The conditions in tidal rivers vary somewhat from those
occurring on the coast. As the crest of the tidal wave passes
the mouth of the river a branch wave is sent up the river. This
wave has first to overcome the water flowing down the river,
which is acting in opposition to it, and in so doing causes a
banking up of the water to such a height that the inclination
of the surface is reversed to an extent sufficient to cause a
tidal current to run up the river. The momentum acquired by the
water passing up-stream carries it to a higher level towards
the head of the river than at the mouth, and, similarly, in
returning, the water flowing down the river gains sufficient
impetus to scoop out the water at the mouth and form a low
water below that in the sea adjoining. Owing to a flow of
upland water down a river the ebb lasts longer than the flood
tide by a period, increasing in length as the distance from the
mouth of the river increases; and, similarly to the sea, the
current may continue to run down a river after the tide has
turned and the level of the water is rising. The momentum of
the tide running up the centre of the river is in excess of
that along the banks, so that the current changes near the
shore before it does in the middle, and, as the sea water is of
greater specific gravity than the fresh, weighing 64 lb per
cubic foot against 62-1/2 lb, it flows up the bed of the river