The Hartford republican. (Hartford, Ky.) 18??-1926, 1921-05-20

|
Of pipe buried in the ground and half
full Of water, Prof. Nicholson has
eliminated disturbances due to wind
and weather and thus has measured
these changes in level with extraor-
binary accuracy. In a lake 200 miles
across the tide would rise half an
inch above the mean level and fall as
low, giving range of one inch. Tides
of this sort have been detected in
the Great Lakes by taking the aver
age of readings of the water level
on many days at the theoretical time
| of high and low tide and so eliminat-
ing the effects of wind and weather
which quite overwhelm the minute
tides on any given day but average
out" in the mean of years of obser-
walton. In seas like the Mediterran-
can the tides are of much the same
nature and have a range of but a
few inches except in a few abnormal
regions.
In large bodies of water the effects
are larger and far more complicated
The oceans cover such large parts of
the earth's surface that the tidal
forces acting on different parts of
them at the same time are quite diff
fercnt. Naturally the continuous
changes due to the earth's rotation
add to these complications. In spite
of these complications, however, the
time and height of tides might be
calculated if the ocean beds were of
uniform depth and regular shape.
But they are actually so irregular
In form and depth and so cut up by
continents and islands that COM
plete mathematical discussion of the
motions of their waters is hopeless.
Yet we know that the moon comes
back to approximately the same posi-
ton as observed from any point on
the earth's surface at regular inter-
walls of "a hours and 81 minutes,
which may be called lunar days.
Therefore the tidal forces due to the
moon repeat themselves over the
ocean at this interval, running thru
their changes twice, once when the
moon is above the horizon and once
when it is below. The rise and fall
of the tides must necessarily follow
suit, rising and falling twice in the
interval of a lunar day.
The tides have a much greater
range in some parts of the ocean
than in others and the time as we'll
az height of high water varies from
place to place. The interval between
successive high waters, however is al
ways exactly one-half Of lunar day.
A tidal pull is also exerted by the
sun but because of its great distance
this is less than the pull of the moon.
The interval between successive high
waters for the solar tides is of course
exactly II hours. Obviously, there
fore, they will alternately get in step
with the lunar tides, reinforcing
them, and out of step, partially coun-
teracting them. The former Of these
effects will evidently occur at the
time of new moon WIN the sun and
moon are close together in the hea-
vens and are pulling on the sea in the
same direction. 1t will also happen
at full moon when the solar tide has
gained half a day on the lunar tide
and reinforces the other high water.
At both of these times we get the
spring tides of large range while mid
way between at the quarters of the
moon, we get the small neap tides.
The two high tides of any day are
not necessarily equal in height nor
are the two low ones necessarily
equal This is because the sun when
north or south of the celestial equa-
tor has at any given hour of the day
an altitude above the horizon differ
ent from its depression below the hor-
lion at the corresponding hour OF the
night. The tidal forces in these two
cases will be nearly the same direc-
ton but will be unequal in amount.
Consequently one high water will be
higher than the other. Similar eff
fects occur in the case of the moon
and we have a 'diurnal inequality"
in the tides. To account fully for
all the changes in the tidal forces
numerous other minor oscillations
must be considered-most Of them
small,
If the height Of the tide has been
observed at a given port at regular in
tervals for a year or so it is possible
to figure out exact information re-
garding the heights and phases OF the
20 or 80 minor tides whose combina-
ton represents the actual situation.
BY calculations based en- such records
the actual course of tides may
be predicted years in advance with
surprising accuracy. These calcula-