Saint Mary's beacon. [volume] (Leonard Town, Md.) 1867-1983, 1871-01-19
THE TIDES.

The phenomena of the Tides were known
to, and were described with considerable
accuracy by, the ancients, even though
their Observations were confined to seas in
which the tidal wave was the least con-
spicuous. Aristotle, among other early
writers, mentions that the tides always
followed the changes OF the moon, and
notes that they were less observable in
small than in large seas. These facts
were mere matters of observation which
could hardly be supposed to have long es-
capped the notice of thinking people. but
it was lot until the discovery by Sir Isaac
Newton of the law of universal gravitation
that the phenomena Of the tides were full
ly and satisfactorily explained.
In order to understand the action of the
moon on our earth as tide-producing
body. We will suppose that the earth con-
sis.s Of smooth sphere, covered all over
with water Of a uniform depth, and that
both bodies are at rest with respect tn
each other: then the attraction of the
moon on the water nearest 10 her Will be
greater than her attraction on the solid
earth. and will cause 4 heaping-up of the
water toward her. This appears natural
enough. but at first sight it does seem
somewhat paradoxical that the water
should also be heapcd up in n direction
opposite to the moon. This Is explained
by the fact that the water on the side away
from the moon is attracted less than the
solid ball of earth, and the latter is, az it
were. drawn away from the water. thus
causing it also to be swelled up, although
not quite to the same extent as on other
side; because. its distance being greater
than in the first case, the moon-attraction
is less.
Now our water-covered globe presents
the appearance of an orange, with its major
axis in the direction Of n line joining the
earth and moon. In this position it would
remain in equilibrium, with constant high
water at two opposite places and low water
on a circle at right angles to and equidis-
tant from these.
Although the sun is at a very much
greater distance from the earth than is the
moon, yet, from its very much greater
mass, it exerts considerable influence in
producing the tides of the ocean, and acts
on the water in precisely the same man
ner as does the moon, producing another
spheroid of equilibrium, With its major
axis in a line joining the earth and sun.
The heaping up Of the waters due to the
sun is two-tifths of that due to the moon,
and il the sun and moon, as seen
from the earth, were separated by
an angle of ninety degrees. We should
have the axes Of our wave spheroids sep
arated BY the quantity. and Gf the three
bodies were at rest with respect 10 each
other) so they would remain to the end
of time But since these conditions do
not obtain in nature. and the earth. moon
and sun, instead of being at rest with re-
gard to each other, are constantly changing
their relative positions and distances (the
earth at the same time revolving on her
axis. these motions never give the water
time to assume the spheroidal shape -
Instead Of this, an imperf ct form of it
travels round the globe IN a lunar day.
which, being about 80 minutes less than
the solar day. causes the tides to GENE
later each successive day by about that
quantity. Ii will be seen from what has
been already said that. when the sun,
moon and earth are in the same straight
line, the solar and lunar waves are COIN
cident-that is, the sun and moon are ac-
ting in concert-and the consequence will
be that the tides about this time will rise
higher and fall lower than they do when
the sun and moon are az right angles 10
each other. When this latter condition
obtains, their action is antagonistic, aud
we h.ve 4 tide that never rises 80 high
nor falls 50 lew as in the former case
That is to say. the highest. or spring.
tides happen at the fun and new moons.
the lowest, or nesp, at frs; and last quar-
tiers.
To calculate the time and height Of
tide. suppcsiny the earth covered uniform
IL with water, would. comparatively speak
ing. be q matter Of ssaill difficulty. But