9
OF ENGINEERING FORMULÆ.
Reduction of Soundings.
R = Total rise of tide in feet.
K = Correction to be subtracted from the sounding to
reduce it to low water.
T = The time between high and low water.
t = The time of taking the soundhig from low water.
u R / 180 t-v v ISO t , „ „
v = - (1 + cos. when -y—exceeds 90°.
11. / 180
K = — (1 — cos, --- ) when it is less than 30°.
Tides.
The sea flows for about 6 hours from south to north in
the northern hemisphere, so that entering the mouths of
rivers it drives the river-waters back towards their source;
after flowing for 6 hours the sea appears to rest for a
quarter of an hour, after which it begins to ebb for 6 hours
more, then there is another seeming pause for a quarter of an
hour.
The sea ebbs and flows twice a day, falling gradually later
by about 48 minutes, each period of flux and reflux being on
an average about 12 hours and 24 minutes.
Tidal Phenomena.
The elevation towards tbe moon slightly exceeds the op-
posite one, and the intensity of the tidal wave diminishes
from the equator towards the poles.
From the action of the sun every day, the sea is twice de-
pressed and twice elevated, following the action of the moon,
but in a less degree.
Spring tides are caused by the combined action of the sun
and moon when both bodies are on the same side of the earth;
neap tides by tbe action of one partly neutralizing that of the
other, when they are on opposite sides of the earth.
The greatest elevations and depressions are not observed
until tbe second or third day after full or new moon.