The Cecil Whig. [volume] (Elkton, Md.) 1841-current, 1875-05-08
WHAT Is THE CAUSE OF TIDES {?
BY S. II. TROwBRIDGE.

BY S. H. TRowBRIDGE.
There are occasional fallacies, which, in
some mysterious way, gain credence in the
minds of men till they finally become &C
cepted as unquestioned facts. Among these
may be mentioned the oft-repeated pro-
verb: 50 1t is always darkest just before
day. and the commonly accepted explana-
ton of the rising of light bodies in a
denser medium. 1t is not true that smoke,
heated air, balloons, ETC., rise because of
their lightness, and then the air rushes in
to take their place; but the air, being
heavier, seeks by gravity the lowest place,
and in so doing crowds up the lighter
bodies Water is said to contract down to
a few degrees of the freezing point, and
then to expand in changing to ice; but it
is probable that the molecules are drawing
closer to one another all the time, and that
the apparent expansion is because the crys-
tails of ice do not fit together exactly, and
hence leave between them interstices filled
with air, and thus occupy more space.
And it is quite possible, if not probable.
that the common explanation of tides fur
nishes still another illustration. With suffi-
Gent credulity, the explained cause of the
tide on the moon side of the earth may
he accepted as somewhat satisfactory; but
there is room for reasonable doubt as re-
CARDS that of the tide opposite the moon.
This luminary is said, in the first case, to
draw the water away from the earth, and
in the second to draw the earth away
from the water. This is considered posi-
BILE because the nearer object will be intlu-
enced more by the moons attraction than
the more distant object, and this difference
| of attractive force, as exerted on the stable
earth and the unstable water, is said to
produce the tides as we observe them.
Attraction varies inversely as the square of
the distance. If we represent the force
with which the moon draws the earth by
ten, the force with which it attracts the
water on the opposite side of the earth will
be about nine and two-thirds. This latter
force is not diminished by the intervening
| earth, and tends to draw the water toward
the moon. The earth, by its attraction,
holds the water to its surface, and its in
| tluence is not lessened when the moon acts
upon it. AS both these forces tend to draw
| the water opposite the moon toward that
luminary, we would reasonably expect a
low, rather than high tide at that point.
It is said that the water remains behind by
its inertia. But as the moon acts con-
stantly upon the earth, and gradually upon
any one point of its surface, the inertia of
the water would be overcome at least as
soon as that of the solid earth, and proba-
bly, sooner, as the water is more free to
yield to the influence of attraction.
Again, the theory rests on the suppose
ton that the attraction of the moon gives
the earth daily motion toward itself; but
this cannot be strictly true, for, if so, the
earth and moon would be continually APP
preaching each other, and we would live in
constant fear of collision, whereas they
maintain uniform mean distance between
them In opposition to this, it is argued
that the dev iations from the tangential
motion of the earth in it's orbit are precise
ly those which the earth would move
through if falling toward the attracting
body unaffected by any other impulse
Whether this is satisfactory, each must de-
eide for himself.
The sun also exerts upon the earth an in
tluence tending to produce tides, which is
about two -fifths as great as that exerted
by the moon. The suns real attraction, OF
course, is much greater than the moons,
but, on account of its greater distance, the
difference between its influence on the
earth and on its aqueous envelope is less.
From the suns influence, we would expect
a tide to follow the sun, as one is said to
follow the moon, and differ from it only in
being smaller; and when the sun and moon
are in quadrature, we should expect, ac-
cording to theory, that there would be four
tides day; two caused by the moon and
two by the sun, whose ma jor axes would be
at right angles to each other. When the
sun and moon are in conjunction we have
the highest tides, because both act together
and in the same direction. When they are
in opposition, we should expect the lowest
tides because they act in opposite direc-
tions and each tends to counterfeit the
effect of the other. But in fact this COM
bination also appears to produce spring
tides.
If the tidal wave is caused by the moon,
and follows her as she apparently makes a
complete circuit of the earth in about %
hours, it must travel at the rate of one
thousand miles per hour, and this is hardly
reconcilable with its mildness and harm
lessness in dashing upon the shore, nol
with Mr.. Airy's law for the velocity OF
tidal waves, which makes it the Same as
that which free body would acquire by
falling from rest, under the action of gray,
ity, through space equal to one-half the
depth of the WATER The Pacific Ocear
is estimated to average 440 fathoms il
depth, and according to this rule the veloc
. ity would be less than 200 miles per hour
or, by a slight change in its application