The sun. [volume] (New York [N.Y.]) 1833-1916, 1899-02-19

Prof. Darwin begins with a definition OF the
phenomenon which forms the subject OF dis
cussion. 1t Is we'll known that the great wave
caused by an earthquake Ia often described in
newspapers as a tidal wave. and the sam term
is not infrequently applied to such q short
series OF enormous fluctuations as Is OCCASION
ally encountered by a ship in the open sea.
Thus used. the adjective "tidal" simply i'm
rlies size and uncommonness consequently
the use OF the term in such a sense Ia inappro-
pirate. In our authors opinion the word
tidal should be employed only when we are
referring to regular and persistent alterations
OF rise and fail in the sea level. Even in this
case the term may sometimes be applied in
too wide a sense. for in many places there Is a
regular alienation OF the wind, which blows
inshore during the day and outshore during
the night with Approximate regularity. and
such breezes alternately raise and depress
the sea level, and thus produce a sort Of tide.
Then, in the tropics. there ia a regularly alter
mating. though small. periodicity in the press
ure OF the atmosphere. which is betrayed by an
oscillation in the height OF the barometer To
such atmospheric pressure the ocean will re-
spond. so that the sea level will fall with a rising
barometer and rise with a falling barometer.
Thus n regularly periodic rise and fall Of the
sea level must result from this cause also,
Again. the melting o. SHOWS in great mountain
ranges and the annual variability in rainfall
and evaporation produce approximately peri
odic changes OF level in the estuaries Of rivers.
and although the period OF these changes 10
very long. extending as they do over the whole
year; yet, from their periodicity. they partake
OF the tidal character.
These changes OF water level are not, how
ever. tides in the proper sense OF the term. and
a true tide can only be adequately defined by
rnfprpnpp tn the pn,1caa ub,oh proJnooj t.
tide. In fact. means a rising and falling Of the
water OF the ocean caused by the attractions Of
the sun and moon. These dual attractions must
certainly act not only on the sea. but also on the
solid earth and. since the earth is not perfectly
rigid or stiff. they must produce an alternating
change in its shape. Even II the earth Is now
so stiff that the changes in its shape escape
detection through their minuteness. yet
such changes of shape must exist. There is
much evidence to show that. in the early
stages OF their histories. the planets consisted
largely. or entirely. of molten rock, which must
have yielded to tidal influences. Prof. Darwin.
consequently, extends the term tide so as to
include such alternating determinations of n
solid and elastic. or Of n molten and plastic
globe. Tho consideration Of these corporeal
tides lends on to some far-reaching astro-
nomical speculations.
MEANWHILE. our author first xamines the
simplest and best known form OF tidal phc-
nomena. We are reminded that, when we are
at the seashore. or on an estuary. We observe
that the water rises and falls nearly twice a
day. To be more exact, the average interval
from one high water to the next is twelve hours
minutes. and so. high water falls
later. according to the clock. by twice twenty
minutes or by fifty minutes on each sue
cessive day. Thus. il high water falls to-day
at noon. 1t will occur tomorrow at ten minutes
to one. Before proceeding further. our author
out that he uses high water and low
water as technical terms. In common
tho level OF water may be
called high or low according to whether IL
Is higher or lower than usual. But, when
the level varies periodically. there are certain
when it is highest and lowest, and
these are referred to in this book as the times
Of high and low water. or Of high and low tide.
The most elementary observations show that
the times of high water have an intimate rela-
tionship to the moons position. On the aver
age. the moon passes the meridian twenty
later on each succeeding day. so that
II high water occurs 80 many hours after the |
moon Is due south on any day it will occur on |
any other day about the same number Of hours |
after the moon was south. This rule il far
rom being exact. for II is found that the in
.erval from the moons passage of the meridian
10 high water differs considerably, according
TO the age OF the moon. It is not only, how
aver. the hour OF high water which changes
rom day today: the height to which the water
rises also varies 80 conspicuously that the fact
'ould not escape the notice OF even the most cas-
lal observer. II would have been needful to CON
suit a clock to discover the law by which the
lour OF high water changes from day to dav.
out. at tho seashore. 1t would he impossible TO
ivoid noticing that some rocks or shoals. which
ire continuously covered by the sea at one part
If a fortnight. are laid bare at another part, II
s. In fact, about full and new moon that the
from low to high water is greatest. and
It the moons first or third quarter that thu
'ange is least. The greater tides are called
and the smaller heaps"
In the fourth chapter OF this volume will h.
ound an Interesting historical sketch OF the
IDEAS conceived by mankind at different epochs
With relation 10 the tides. 1t appears that
'hinese writer's suggested two causes for
his phenomenon: first that water is the blood
4 the earth. and that the tides represent the
OF its pulse: and. secondly. that the
ides arc caused by the earth's breathing. Ko
lung. a writer OF the fourth century Of our
ra. gave a somewhat obscure explanation o,
pring and neap tides. He said that every